staging: vchiq_core: fix service dereference in unlock_service

The service state is dereferenced before BUG_ON and outside of the
spin lock. So in order to avoid possible NULL pointer dereferences or
races move the whole scope at a safer place.

This issue has been found by Cppcheck.

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