PCI: Clean up __pci_find_next_cap_ttl() readability

Refactor the __pci_find_next_cap_ttl() to improve code clarity:

  - Replace magic number 0x40 with PCI_STD_HEADER_SIZEOF.
  - Use ALIGN_DOWN() for position alignment instead of manual bitmask.
  - Extract PCI capability fields via FIELD_GET() with standardized masks.
  - Add necessary headers (linux/align.h).

No functional changes intended.

Signed-off-by: Hans Zhang <18255117159@163.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Acked-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20250813144529.303548-2-18255117159@163.com

Author: Hans Zhang

drivers/pci/pci.c

@@ -9,6 +9,7 @@
  */
 
 #include <linux/acpi.h>
+#include <linux/align.h>
 #include <linux/kernel.h>
 #include <linux/delay.h>
 #include <linux/dmi.h>
@@ -432,17 +433,17 @@ static u8 __pci_find_next_cap_ttl(struct pci_bus *bus, unsigned int devfn,
 	pci_bus_read_config_byte(bus, devfn, pos, &pos);
 
 	while ((*ttl)--) {
-		if (pos < 0x40)
+		if (pos < PCI_STD_HEADER_SIZEOF)
 			break;
-		pos &= ~3;
+		pos = ALIGN_DOWN(pos, 4);
 		pci_bus_read_config_word(bus, devfn, pos, &ent);
 
-		id = ent & 0xff;
+		id = FIELD_GET(PCI_CAP_ID_MASK, ent);
 		if (id == 0xff)
 			break;
 		if (id == cap)
 			return pos;
-		pos = (ent >> 8);
+		pos = FIELD_GET(PCI_CAP_LIST_NEXT_MASK, ent);
 	}
 	return 0;
 }

include/uapi/linux/pci_regs.h

@@ -207,6 +207,9 @@
 
 /* Capability lists */
 
+#define PCI_CAP_ID_MASK		0x00ff	/* Capability ID mask */
+#define PCI_CAP_LIST_NEXT_MASK	0xff00	/* Next Capability Pointer mask */
+
 #define PCI_CAP_LIST_ID		0	/* Capability ID */
 #define  PCI_CAP_ID_PM		0x01	/* Power Management */
 #define  PCI_CAP_ID_AGP		0x02	/* Accelerated Graphics Port */