scsi: raid_attrs: fix unused variable warning

I ran into a new warning on randconfig kernels:

drivers/scsi/raid_class.c: In function 'raid_match':
drivers/scsi/raid_class.c:64:24: error: unused variable 'i' [-Werror=unused-variable]

This looks like a very old problem that for some reason was very hard to
run into, but it is very easy to fix, by replacing the incorrect #ifdef
with a simpler IS_ENABLED() check.

Fixes: fac829fd

 ("[SCSI] raid_attrs: fix dependency problems")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>