V4L/DVB: az6027: az6027_read_mac_addr is currently unused

drivers/media/dvb/dvb-usb/az6027.c:759: warning: ‘az6027_read_mac_addr’ defined but not used

While there's some code that uses it, it is currently commented. So, comment also
the function itself.

CC: Manu Abraham <abraham.manu@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>