bnxt_en: eliminate the compile warning in bnxt_request_irq due to CONFIG_RFS_ACCEL

I received a kernel-test-bot report[1] that shows the
[-Wunused-but-set-variable] warning. Since the previous commit I made, as
the 'Fixes' tag shows, gives users an option to turn on and off the
CONFIG_RFS_ACCEL, the issue then can be discovered and reproduced with
GCC specifically.

Like Simon and Jakub suggested, use fewer #ifdefs which leads to fewer
bugs.

[1]
All warnings (new ones prefixed by >>):

   drivers/net/ethernet/broadcom/bnxt/bnxt.c: In function 'bnxt_request_irq':
>> drivers/net/ethernet/broadcom/bnxt/bnxt.c:10703:9: warning: variable 'j' set but not used [-Wunused-but-set-variable]
   10703 |  int i, j, rc = 0;
         |         ^

Fixes: 9b6a30f ("net: allow rps/rfs related configs to be switched")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506282102.x1tXt0qz-lkp@intel.com/
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: JasonXing

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -11607,11 +11607,9 @@ static void bnxt_free_irq(struct bnxt *bp)
 
 static int bnxt_request_irq(struct bnxt *bp)
 {
+	struct cpu_rmap *rmap = NULL;
 	int i, j, rc = 0;
 	unsigned long flags = 0;
-#ifdef CONFIG_RFS_ACCEL
-	struct cpu_rmap *rmap;
-#endif
 
 	rc = bnxt_setup_int_mode(bp);
 	if (rc) {
@@ -11632,15 +11630,15 @@ static int bnxt_request_irq(struct bnxt *bp)
 		int map_idx = bnxt_cp_num_to_irq_num(bp, i);
 		struct bnxt_irq *irq = &bp->irq_tbl[map_idx];
 
-#ifdef CONFIG_RFS_ACCEL
-		if (rmap && bp->bnapi[i]->rx_ring) {
+		if (IS_ENABLED(CONFIG_RFS_ACCEL) &&
+		    rmap && bp->bnapi[i]->rx_ring) {
 			rc = irq_cpu_rmap_add(rmap, irq->vector);
 			if (rc)
 				netdev_warn(bp->dev, "failed adding irq rmap for ring %d\n",
 					    j);
 			j++;
 		}
-#endif
+
 		rc = request_irq(irq->vector, irq->handler, flags, irq->name,
 				 bp->bnapi[i]);
 		if (rc)