ALSA: emu10k1: Simplify firmware loader code

The EMU1010 support in emu10k1 driver has two request_firmware()
calls, one for the main board and one for the dock.  Both call
patterns are fairly similar, and we can simplify it by introducing a
helper function and a table instead of the open switch/case.

Signed-off-by: Takashi Iwai <tiwai@suse.de>