Bus Pirate driver: Small assorted fixes.

Fixes are:
- Discard any stale data from the previous connection.
- Disable CR/LF translation on the (virtual USB) serial port.
- Increase the average USB packet size. The 1 KiB buffer was underutilised.
- Option "buspirate_speed fast" now works out of the box.
- Some extra comments, error checking, etc.

Change-Id: I72c49d943a8ce9c5e5d1644ef90cb1482f93c618
Signed-off-by: R. Diez <rdiezmail-openocd@yahoo.de>
Reviewed-on: http://openocd.zylin.com/1437
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>