[PATCH] USB serial: Converts port semaphore to mutexes.

The usbserial's port semaphore used to synchronize serial_open()
and serial_close() are strict mutexes, convert them to the mutex
implementation.

Signed-off-by: Luiz Capitulino <lcapitulino@mandriva.com.br>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>