[media] cx231xx: add wrapper to get the i2c_adapter pointer

This is a preparation for mapping I2C_1_MUX_1 and I2C_1_MUX_3 later to the seperate
muxed i2c adapters.

Map mux adapters to I2C_1 for now.

Add local variables for i2c_adapters in dvb_init to get line lengths
shorter.

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>