PCI: Make minimum bridge window alignment reference more obvious

Calculations related to bridge window size contain literal 20 that is the
minimum alignment for a bridge window. Make the code more obvious by
converting the literal 20 to __ffs(SZ_1M).

Link: https://lore.kernel.org/r/20240507102523.57320-8-ilpo.jarvinen@linux.intel.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
[bhelgaas: squash https://lore.kernel.org/r/20240612093250.17544-1-ilpo.jarvinen@linux.intel.com]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>

Author: ij-intel

drivers/pci/setup-bus.c

@@ -14,13 +14,15 @@
  *	     tighter packing. Prefetchable range support.
  */
 
+#include <linux/bitops.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/pci.h>
 #include <linux/errno.h>
 #include <linux/ioport.h>
 #include <linux/cache.h>
+#include <linux/sizes.h>
 #include <linux/slab.h>
 #include <linux/acpi.h>
 #include "pci.h"
@@ -957,7 +959,7 @@ static inline resource_size_t calculate_mem_align(resource_size_t *aligns,
 	for (order = 0; order <= max_order; order++) {
 		resource_size_t align1 = 1;
 
-		align1 <<= (order + 20);
+		align1 <<= order + __ffs(SZ_1M);
 
 		if (!align)
 			min_align = align1;
@@ -1047,7 +1049,7 @@ static int pbus_size_mem(struct pci_bus *bus, unsigned long mask,
 			 * resources.
 			 */
 			align = pci_resource_alignment(dev, r);
-			order = __ffs(align) - 20;
+			order = __ffs(align) - __ffs(SZ_1M);
 			if (order < 0)
 				order = 0;
 			if (order >= ARRAY_SIZE(aligns)) {