media: dvb: Add check on sp8870_readreg

In sp8870_set_frontend_parameters, the function sp8870_readreg
may return an error when i2c_transfer fails. The fix checks for
this error and returns upstream consistent with other invocations.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>