ALSA: es18xx: Fix assignment in if condition

ISA ES18xx driver code contains lots of 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/20210608140540.17885-11-tiwai@suse.de


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