[media] tuner/xc2028: Add I2C flush callback

When loading the firmware, complete each chunk by sending an I2C flush
command to the frontend. Some devices like the tm6000 seem to require
this to properly flush the I2C buffers.

The current code in tm6000 executes the flush command once after each
I2C transfer, which slows down the firmware loading especially when
loading large BASE type images.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>