[media] em28xx: fix the i2c adapter functionality flags

I2C_FUNC_SMBUS_EMUL includes flag I2C_FUNC_SMBUS_WRITE_BLOCK_DATA which signals
that up to 31 data bytes can be written to the ic2 client.
But the EM2800 supports only i2c messages with max. 4 data bytes.
I2C_FUNC_IC2 should be set if a master_xfer function pointer is provided in
struct i2c_algorithm.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>