scsi: mvumi: use __maybe_unused to hide pm functions

The mvumi scsi hides the references to its suspend/resume functions in
an #ifdef but does not hide the implementation the same way:

drivers/scsi/mvumi.c:2632:12: error: 'mvumi_suspend' defined but not used [-Werror=unused-function]
drivers/scsi/mvumi.c:2651:12: error: 'mvumi_resume' defined but not used [-Werror=unused-function]

This adds __maybe_unused annotations so the compiler knows it can
silently drop them instead of warning, while avoiding the addition of
another #ifdef.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>