[media] em28xx: Prepare to support 2 different I2C buses

Newer em28xx devices have 2 buses. Change the logic to allow
using both buses.
This patch was generated by this small script:
for i in drivers/media/usb/em28xx/*.c; do
	sed 's,->i2c_adap,->i2c_adap[dev->def_i2c_bus],g;s,->i2c_client,->i2c_client[dev->def_i2c_bus],'
done
Of course, em28xx.h needed manual edit.

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