infiniband/ulp/ipoib: remove pkey_mutex

The last user of pkey_mutex was removed in db84f880

 ("IB/ipoib: Use
P_Key change event instead of P_Key polling mechanism") but the lock
remained.
This patch removes it.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>