USB: debug port converter does not accept more than 8 byte packets

USB debug port only supports 8 byte rx/tx packets. Although spec implies that
"if a packet larger than eight bytes is received from the remote computer, the
device must break the larger packet into eight-byte packets before sending the
data to the Debug Port", the real PLX NET20DC device does not handle it right -
data is corrupted on debug port end if serial interface sends >8 byte urbs.
Patch below fixes the issue by limiting tx urb to 8 byte.

Signed off by: Aleks Gorelov <dared1st@yahoo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>