hw/pci-host/apb.c: Avoid shifting left into sign bit

Add U suffix to avoid undefined behaviour.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/pci-host/apb.c

@@ -58,11 +58,11 @@ do { printf("APB: " fmt , ## __VA_ARGS__); } while (0)
 #define PBM_PCI_IMR_MASK    0x7fffffff
 #define PBM_PCI_IMR_ENABLED 0x80000000
 
-#define POR          (1 << 31)
-#define SOFT_POR     (1 << 30)
-#define SOFT_XIR     (1 << 29)
-#define BTN_POR      (1 << 28)
-#define BTN_XIR      (1 << 27)
+#define POR          (1U << 31)
+#define SOFT_POR     (1U << 30)
+#define SOFT_XIR     (1U << 29)
+#define BTN_POR      (1U << 28)
+#define BTN_XIR      (1U << 27)
 #define RESET_MASK   0xf8000000
 #define RESET_WCMASK 0x98000000
 #define RESET_WMASK  0x60000000