drivers/serial.c: Fix some race conditions. Some bad things code happen if we...

drivers/serial.c: Fix some race conditions.  Some bad things code happen if we lost a USB connection at certain times.

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@5596 42af7a65-404d-4744-a932-0658087f49c3