hw/omap_gpio: Fix infinite recursion when doing 8/16 bit reads

Fix a long-standing bug which meant that any attempt to do an
8 or 16 bit read from the OMAP GPIO module would cause qemu to
crash due to an infinite recursion.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
