ALSA: seq: Fix assignment in if condition

There are lots of places doing assignments in if condition in ALSA
sequencer core, 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/20210608140540.17885-57-tiwai@suse.de


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