Staging: comedi: dt9812: Use mutex instead of semaphore

Replace binary semaphore with mutex. Mutex also gives better performance
than semaphore.

Signed-off-by: Ksenija Stanojevic <ksenija.stanojevic@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>