scsi: acornscsi: mark calc_sync_xfer as __maybe_unused

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>