scsi: acornscsi: mark calc_sync_xfer as __maybe_unused

The calc_sync_xfer function is only used if CONFIG_SCSI_ACORNSCSI_SYNC
is set, otherwise we get a compiler warning:

scsi/arm/acornscsi.c:680:15: warning: 'calc_sync_xfer' defined but not used [-Wunused-function]
 unsigned char calc_sync_xfer(unsigned int period, unsigned int offset)

This marks the function as __maybe_unused to shut up the warning and
silently drop the function in the object code when there is no caller.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/arm/acornscsi.c

@@ -677,7 +677,8 @@ int round_period(unsigned int period)
  * Copyright: Copyright (c) 1996 John Shifflett, GeoLog Consulting
  */
 static
-unsigned char calc_sync_xfer(unsigned int period, unsigned int offset)
+unsigned char __maybe_unused calc_sync_xfer(unsigned int period,
+					    unsigned int offset)
 {
     return sync_xfer_table[round_period(period)].reg_value |
 		((offset < SDTR_SIZE) ? offset : SDTR_SIZE);