ALSA: isa: Move snd_legacy_find_free_ioport to initval.h

Move snd_legacy_find_free_ioport() function back to initval.h as it is used
by two drivers.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>