This commit solves a problem which causes data loss while sending data via...

This commit solves a problem which causes data loss while sending data via USB. This problem is caused by an incorrect handling of the endpoint state in the USB driver sam_usbdevhs.  This leads under some circumstances to situations in which an DMA transfer is setup while a previous DMA transfer is currently active. Amongst other things I introduced the new endpoint state USBHS_EPSTATE_SENDING_DMA for the fix.

To reproduce the problem, I used a program which send as many data as possible via a CDC/ACM device and verified the received data on the PC.