scsi: aic7xxx: Fix 'amount_xferred' set but not used issue

'amount_xferred' is used, but only in certain circumstances.  Place the
same stipulations on the defining/allocating of 'amount_xferred' as is
placed when using it.

We've been careful not to change any of the ordering semantics here.

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

 drivers/scsi/aic7xxx/aic7xxx_osm.c: In function ‘ahc_done’:
 drivers/scsi/aic7xxx/aic7xxx_osm.c:1725:12: warning: variable ‘amount_xferred’ set but not used [-Wunused-but-set-variable]
 1725 | uint32_t amount_xferred;
 | ^~~~~~~~~~~~~~

Link: https://lore.kernel.org/r/20200713074645.126138-22-lee.jones@linaro.org
Cc: Hannes Reinecke <hare@suse.com>
Cc: "Daniel M. Eischen" <deischen@iworks.InterWorks.org>
Cc: Doug Ledford <dledford@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/aic7xxx/aic7xxx_osm.c

@@ -1711,10 +1711,12 @@ ahc_done(struct ahc_softc *ahc, struct scb *scb)
 	 */
 	cmd->sense_buffer[0] = 0;
 	if (ahc_get_transaction_status(scb) == CAM_REQ_INPROG) {
+#ifdef AHC_REPORT_UNDERFLOWS
 		uint32_t amount_xferred;
 
 		amount_xferred =
 		    ahc_get_transfer_length(scb) - ahc_get_residual(scb);
+#endif
 		if ((scb->flags & SCB_TRANSMISSION_ERROR) != 0) {
 #ifdef AHC_DEBUG
 			if ((ahc_debug & AHC_SHOW_MISC) != 0) {