pci/shpc: fix signed integer overflow

clang undefined behaviour sanitizer reports:
> hw/pci/shpc.c:162:27: runtime error: left shift of 1 by 31 places
> cannot be represented in type 'int'

Caused by the usual lack of a 'U' qualifier on a constant 1 being
shifted left. Fix it up.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/pci/shpc.c

@@ -159,7 +159,7 @@ static void shpc_interrupt_update(PCIDevice *d)
     for (slot = 0; slot < shpc->nslots; ++slot) {
         uint8_t event = shpc->config[SHPC_SLOT_EVENT_LATCH(slot)];
         uint8_t disable = shpc->config[SHPC_SLOT_EVENT_SERR_INT_DIS(d, slot)];
-        uint32_t mask = 1 << SHPC_IDX_TO_LOGICAL(slot);
+        uint32_t mask = 1U << SHPC_IDX_TO_LOGICAL(slot);
         if (event & ~disable) {
             int_locator |= mask;
         }