V4L/DVB (13976): [STV090x] Added mutex protection around tuner I2C access.

With this change it is possible to have the same I2C address for both tuners.

Signed-off-by: Andreas Regel <andreas.regel@gmx.de>
Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>