ALSA: firewire: use 'GPL' string for module license contributed by Takashi Sakamoto

In MODULE_LICENSE macro, "GPL" string obsoletes "GPL v2" string by a
commit bf7fbeea

 ("module: Cure the MODULE_LICENSE "GPL" vs. "GPL v2"
bogosity").

This commit uses the preferable expression.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20230611144445.221529-2-o-takashi@sakamocchi.jp


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