V4L/DVB (13986): [STV090x] Disable I2C gate on error

The I2C gate must also be disabled, if a tuner command failed.
Otherwise the tuner mutex would be locked forever.

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