ALSA: serial: Fix assignment in if condition

A few ALSA serial drivers contain 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/20210608140540.17885-63-tiwai@suse.de


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