ALSA: opl4: Use IS_REACHABLE()

Rewrite the complex ifdef condition with IS_REACHABLE().
The ifdef in opl4_local.h was without defined(MODLE) check, but this
is likely the oversight.  Use IS_REACHABLE() here as well.

Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>