ALSA: poewrmac: Fix assignment in if condition

PPC powermac driver code contains a few assignments in if condition,
which is a bad coding style that may confuse readers and occasionally
lead to bugs.

This patch is merely for coding-style fixes, no functional changes.

Link: https://lore.kernel.org/r/20210608140540.17885-65-tiwai@suse.de


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