[WATCHDOG] omap_wdt.c: sync linux-omap changes

These are changes that have been sitting in linux-omap
and were never sent upstream.

Hopefully, it'll never happen again at least for this
driver.

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>