gpio/omap: cleanup _set_gpio_wakeup(), remove ifdefs

Make _set_gpio_wakeup() generic by removing ifdefs.  Code for the
various SoCs/bank-methods was already the same, except for the
non-wakeup GPIO checking.  But that flag is set on a per-SoC basis, so
can be used for all SoCs.

While here, use dev_err() and remove GPIO bank calculation assumption
based on subtracting bank pointers.

Signed-off-by: Kevin Hilman <khilman@ti.com>