[ALSA] oxygen: make SPI configuration configurable

Add a field to the model structure so that it is possible to have a card
where the SPI outputs 4 and 5 are used for an EEPROM.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>