ALSA: hdsp - Fix detection for RME RPM/Multiface/Digiface ioboxes

The current iobox detection code reportedly fails for various users, so
simply do what the Win32 driver does instead.

Patch originally by Karl Grill <kgrill@chello.at> and then modified to
comply with kernel coding guidelines + current HEAD.

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