[media] ds3000: remove useless 'locking'

Since b9bf2eaf, the function
ds3000_firmware_ondemand() is called only once during init. This
locking scheme may have been useful when the firmware was loaded at
each tune.
Furthermore, it looks like this 'lock' was put in to prevent concurrent
access (and not recursion as the comments suggest). However, this open-
coded mechanism is anything but race-free and should have used a proper
mutex.

Signed-off-by: Rémi Cardona <remi.cardona@smartjog.com>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>