hw/openrisc: Avoid undefined shift in openrisc_pic_cpu_handler()

In C99 signed shift (1 << 31) is undefined behavior, since the result
exceeds INT_MAX.  Use 1U instead and move the shift after the check.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Acked-by: Jia Liu <proljc@gmail.com>
