[media] media: venus: hfi: fix mutex unlock

This fixed a warning when build driver with gcc7:

drivers/media/platform/qcom/venus/hfi.c:171
hfi_core_ping() warn: inconsistent returns 'mutex:&core->lock'.
  Locked on:   line 159
  Unlocked on: line 171

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>