media: dvb-frontends/stv6111: handle gate_ctrl errors

When a parent (demod) driver encounters and signals a problem with
gate_ctrl(), don't blindly continue poking the I2C bus.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <rascobie@slingshot.co.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>