staging: brcm80211: use mutex instead of semaphore in dhd_linux.c

The semaphore proto_sem has been replaced with mutex proto_block
which lock certain code paths for one thread.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>