ALSA: sb8: Add a comment note regarding an unused pointer

The field "fm_res" of "struct snd_sb8" is never used/dereferenced
throughout the sb8.c code. Therefore there is no need for any null value
check after the "request_region()".

Add a comment note to make developers know about this and prevent any
"NULL check" patches on this part of code.

Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Atul Gopinathan <atulgopinathan@gmail.com>
Link: https://lore.kernel.org/r/20210503115736.2104747-36-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>