[media] cx231xx-i2c: handle errors with cx231xx_get_i2c_adap()

[media] cx231xx-i2c: handle errors with cx231xx_get_i2c_adap()

The cx231xx_get_i2c_adap() function should return the I2C
adapter that will be used to talk with a device. It should never
be NULL, as otherwise the driver will try to dereference a
null pointer.

We might instead fix the callers, but if this condition
ever happens, it is really a driver bug, because i2c_port
should always be a value from enum CX231XX_I2C_MASTER_PORT.

Found when checking the code due to this bug:

[   39.769021] BUG: unable to handle kernel NULL pointer dereference at 0000000000000002
[   39.769105] IP: [<ffffffff81638393>] i2c_master_send+0x13/0x70

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>