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: NOlli Salonen <olli.salonen@iki.fi>
Signed-off-by: NAntti Palosaari <crope@iki.fi>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>