[media] az6007: Simplify the read/write logic

This patch introduces no functional changes. It basically defines
a macro for each different req found at the driver, and cleans the
code to use them, making easier to understand the code.

With regards to the IR handling code, although the original code
doesn't define what's the request, it is clear, from the USB logs,
that 0xc5 is for IR polling.

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