V4L/DVB (7633): au8522: consolidate mse2snr_lookup functions

consolidate au8522_qam256_mse2snr_lookup, au8522_qam64_mse2snr_lookup and
au8522_vsb_mse2snr_lookup into a single function, au8522_mse2snr_lookup.

Pass the mse2snr table into au8522_mse2snr_lookup depending on the
modulation type.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>