scsi: fdomain: Mark 'fdomain_pm_ops' as __maybe_unused

Not all source files which include 'fdomain.h' make use of 'fdomain_pm_ops'
leaving them defined but unused.  Mark it as __maybe_unused to tell the
compiler this is not only acceptable, but expected.

Fixes the following W=1 kernel build warning(s):

 In file included from drivers/scsi/pcmcia/fdomain_cs.c:16:
 drivers/scsi/fdomain.h:106:32: warning: ‘fdomain_pm_ops’ defined but not used [-Wunused-const-variable=]
 106 | static const struct dev_pm_ops fdomain_pm_ops;
 | ^~~~~~~~~~~~~~

Link: https://lore.kernel.org/r/20200707140055.2956235-4-lee.jones@linaro.orgSigned-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/fdomain.h

@@ -103,7 +103,7 @@ enum {
 #define REG_FIFO_COUNT		14	/* R: FIFO Data Count */
 
 #ifdef CONFIG_PM_SLEEP
-static const struct dev_pm_ops fdomain_pm_ops;
+static const struct dev_pm_ops __maybe_unused fdomain_pm_ops;
 #define FDOMAIN_PM_OPS	(&fdomain_pm_ops)
 #else
 #define FDOMAIN_PM_OPS	NULL