s390/stp: unify stp_work_mutex and clock_sync_mutex

No need to have two mutexes, and while at it rename it to
stp_mutex.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>