watchdog: omap_wdt.c: fix the WDIOC_GETBOOTSTATUS ioctl if not implemented.

WDIOC_GETBOOTSTATUS ioctl is imlemented for cpu_is_omap16xx and cpu_is_omap24xx
cpus only. For other cpus it falls through to WDIOC_KEEPALIVE.
This patch prevents the fall through.

Cc: sricharan <r.sricharan@ti.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>