[media] dib9000: fix explicit lock mismatches

There are several error paths, where &state->platform.risc.mem_mbx_lock
is not unlocked. The patch fixes it.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>