media: dvb: Add check on sp8870_readreg return

The function sp8870_readreg returns a negative value when i2c_transfer
fails so properly check for this and return the error if it happens.

Cc: Sean Young <sean@mess.org>
Cc: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Alaa Emad <alaaemadhossney.ae@gmail.com>
Link: https://lore.kernel.org/r/20210503115736.2104747-60-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>