hdspmixer: Lower nesting depth in label assignment

hdspmixer: Lower nesting depth in label assignment

This fix is mostly cosmetic, the long if-then-else-if-chain was rather
confusing. To improve code readability, use simple if-then statements
and leave the function as soon as a match is found.

Clear code would probably use a two-dimensional map, but it seems rather
overengineered.

Last but not least, the non-matching case was augmented by assigning a
generic "1", "2", "3" .. channel naming schema to prevent hdspmixer from
crashing.

Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>