[media] stv090x: Optional external lock routine

Card driver may supply its own lock routine now.

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>