[media] em28xx: add helper function for reading data blocks from i2c clients

Add a helper function for reading data blocks from i2c devices with 8 or 16 bit
address width and 8 bit register width.
This allows us to reduce the size of new code added by the following patches.
Works only for devices with activated register auto incrementation.

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