V4L/DVB (5169): Pvrusb2: Use macro names for FX2 commands

This is a maintainability cleanup; use nice names for all the FX2
commands instead of raw bytes.  This way we can easily find where we
issue FX commands.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>