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>
