[media] em28xx: Only change I2C bus inside em28xx-i2c

There's currently a bug on em28xx-i2c that makes it write the
wrong values to register 06, that controlls the I2C bus speed
and bus.

Fix it to change only the I2C bus flag.

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