hw/gpio/aspeed_gpio: Avoid shift into sign bit

hw/gpio/aspeed_gpio: Avoid shift into sign bit

In aspeed_gpio_update() we calculate "mask = 1 << gpio", where
gpio can be between 0 and 31. Coverity complains about this
because 1 << 31 won't fit in a signed integer.

For QEMU this isn't an error because we enable -fwrapv,
but we can keep Coverity happy by doing the shift on
unsigned numbers.

Resolves: Coverity CID 1547742
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cédric Le Goater <clg@redhat.com>