media: dvb-frontends/stv0910: cleanup I2C access functions

write_reg() and i2c_write_reg16() only act as a proxy to i2c_write(), which
isn't called from anywhere else throughout the driver. Clean this up by
moving the message setup and the i2c_transfer() into write_reg() so it
becomes the only I2C write function. While touching those parts, fix the
error codes from EREMOTEIO to EIO.

The I2C cleanup is picked from the upstream dddvb.

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>