change default size of CDC/ACM serial buffer sizes

drivers/usbdev/Kconfig

@@ -244,6 +244,10 @@ config PL2303_BULKIN_REQLEN
 		on the other hand, request buffer size is always the same as the
 		maxpacket size.
 
+		There is also no reason from PL2303_BULKIN_REQLEN to be greater
+		than PL2303_TXBUFSIZE-1, since a request larger than the TX
+		buffer can never be sent.
+
 config PL2303_RXBUFSIZE
 	int "Receive buffer size"
 	default 513 if USBDEV_DUALSPEED
@@ -257,14 +261,14 @@ config PL2303_RXBUFSIZE
 
 config PL2303_TXBUFSIZE
 	int "Transmit buffer size"
-	default 513 if USBDEV_DUALSPEED
-	default 257 if !USBDEV_DUALSPEED
+	default 767 if USBDEV_DUALSPEED
+	default 193 if !USBDEV_DUALSPEED
 	---help---
 		Size of the serial transmit buffers.  The actual amount of data that
 		can be held in the buffer is this number minus one due to the way
-		that the circular buffer is managed.  So a TX buffer size of 257
-		will hold four full-speed, 64 byte packets; a buffer size of 513
-		will hold one high-speed, 512 byte packet.
+		that the circular buffer is managed.  So a TX buffer size of 767
+		will hold one request of size 768; a buffer size of 193 will hold
+		two requests of size 96 bytes.
 
 config PL2303_VENDORID
 	hex "Vendor ID"
@@ -429,6 +433,10 @@ config CDCACM_BULKIN_REQLEN
 		on the other hand, request buffer size is always the same as the
 		maxpacket size.
 
+		There is also no reason from CDCACM_BULKIN_REQLEN to be greater
+		than CDCACM_TXBUFSIZE-1, since a request larger than the TX
+		buffer can never be sent.
+
 config CDCACM_RXBUFSIZE
 	int "Receive buffer size"
 	default 513 if USBDEV_DUALSPEED
@@ -442,14 +450,14 @@ config CDCACM_RXBUFSIZE
 
 config CDCACM_TXBUFSIZE
 	int "Transmit buffer size"
-	default 513 if USBDEV_DUALSPEED
-	default 257 if !USBDEV_DUALSPEED
+	default 767 if USBDEV_DUALSPEED
+	default 193 if !USBDEV_DUALSPEED
 	---help---
 		Size of the serial transmit buffers.  The actual amount of data that
 		can be held in the buffer is this number minus one due to the way
-		that the circular buffer is managed.  So a TX buffer size of 257
-		will hold four full-speed, 64 byte packets; a buffer size of 513
-		will hold one high-speed, 512 byte packet.
+		that the circular buffer is managed.  So a TX buffer size of 767
+		will hold one request of size 768; a buffer size of 193 will hold
+		two requests of size 96 bytes.
 
 config CDCACM_VENDORID
 	hex "Vendor ID"