ALSA: hdspm - Refactor serial number to avoid code duplication

The serial number is used multiple times in hdspm.c. Since it belongs
to the card, let's store it in struct hdspm and refer to it whenever
necessary.

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