[media] drx-j: Split firmware size check from the main routine

The firmware upload routine is already complex enough. Split the
first loop that verifies the firmware size into a separate routine,
making the code more readable.

Acked-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>