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/aic79xx_osm.c: In function ‘ahd_done’:
 drivers/scsi/aic7xxx/aic79xx_osm.c:1796:12: warning: variable ‘amount_xferred’ set but not used [-Wunused-but-set-variable]

Link: https://lore.kernel.org/r/20200713080001.128044-24-lee.jones@linaro.org
Cc: Hannes Reinecke <hare@suse.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/aic79xx_osm.c

@@ -1787,10 +1787,12 @@ ahd_done(struct ahd_softc *ahd, struct scb *scb)
 	 */
 	cmd->sense_buffer[0] = 0;
 	if (ahd_get_transaction_status(scb) == CAM_REQ_INPROG) {
+#ifdef AHD_REPORT_UNDERFLOWS
 		uint32_t amount_xferred;
 
 		amount_xferred =
 		    ahd_get_transfer_length(scb) - ahd_get_residual(scb);
+#endif
 		if ((scb->flags & SCB_TRANSMISSION_ERROR) != 0) {
 #ifdef AHD_DEBUG
 			if ((ahd_debug & AHD_SHOW_MISC) != 0) {