ALSA: opl3: Fix assignment in if condition

OPL3 helper 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/20210608140540.17885-62-tiwai@suse.de


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