Clean up the locking in usb_linux.cpp.

tsan complained that usb_bulk_write accesses usb_handle members outside
a lock. Fix that, but by moving everything over to C++11 locking.

Note that the old code was checking whether pthread_cond_timedwait returned
a negative value, which it will never do --- it will signal timeout (or
any other error) by returning a positive errno value. The rewrite does
what they appeared to intend to do (break out on timeout), rather than
what they actually did (keep trying forever).

Bug: http://b/22598587
Change-Id: Iab6869ffed4874143a7da97193d6b09e34cf2933