[media] rtl28xxu: add I2C read without write

Add support for I2C read operation without a preceeding write.

While here, change the error code to EOPNOTSUPP in case an
unsupported I2C operation is attempted.

Signed-off-by: Olli Salonen <olli.salonen@iki.fi>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>