media: siano: use DEFINE_MUTEX() for mutex lock

mutex lock can be initialized with DEFINE_MUTEX() rather than
explicitly calling mutex_init().

Signed-off-by: Muhammad Usama Anjum <musamaanjum@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>