staging: vchiq_arm: Fix unlocked access to dequeue_pending

The dequeue_pending flag wasn't protected by a spinlock in the
service_callback. So fix this to make it safe.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>