x86: pci_io_cfg: Make constant unsigned to fix out of bounds shift

Fix the error below when running a coreboot image built with
`CONFIG_UBSAN=y`.

    PCI: pci_scan_bus for bus 00
    shift out of bounds src/arch/x86/include/arch/pci_io_cfg.h:13:20
    ubsan: unrecoverable error.

GCC with `-fsanitize=shift` also flags this:

    runtime error: left shift of 1 by 31 places cannot be represented in type 'int'

So, make the constant unsigned.

TEST=emulation/qemu-i440fx with `CONFIG_UBSAN=y` stops later with
         [ERROR]  unaligned access src/lib/rmodule.c:152:27
	 [EMERG]  ubsan: unrecoverable error.
Change-Id: Ib05d225ab9f22078d765009b4ee6ef0c63231eed
Found-by: UBSAN
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51292
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/arch/x86/include/arch/pci_io_cfg.h

@@ -13,7 +13,7 @@
 static __always_inline
 uint32_t pci_io_encode_addr(pci_devfn_t dev, uint16_t reg)
 {
-	uint32_t addr = 1 << 31;
+	uint32_t addr = 1U << 31;
 
 	addr |= dev >> 4;
 	addr |= reg & 0xfc;