V4L/DVB (11342): au0828: better document i2c registers

Change the #define entries for the i2c registers to be more meaningful, and
document the valid values for the registers.  Note that this changeset makes
*no* functional changes to the code.

Signed-off-by: Devin Heitmueller <dheitmueller@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>