media: ngene: Replace semaphore cmd_mutex with mutex

The semaphore 'cmd_mutex' is used as a simple mutex, so
it should be written as one. Also, replace down with
mutex_destroy to ensure sane state when ngene_stop is
called.

Signed-off-by: Binoy Jayan <binoy.jayan@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>