ARM: OMAP: minor gpio bugfixes

Minor GPIO fixes:

 - If get_gpio_bank() fails, then BUG() out.

 - In omap_set_gpio_debounce():
    * protect the read/modify/write with the relevant spinlock
    * make the omap3 clock ops pass "sparse" checking

Except for the spinlock problem, these were reported through "make".

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>