watchdog: omap_wdt.c: Fix the mismatch of pm_runtime enable and disable

Currently the watchdog driver calls the pm_runtime_enable and never
the disable. This may cause a warning when pm_runtime_enable
checks for the count match.

Also fixes the error

/build/watchdog # insmod omap_wdt.ko
[   44.999389] omap_wdt omap_wdt: Unbalanced pm_runtime_enable!
[   45.011047] OMAP Watchdog Timer Rev 0x00: initial timeout 60 sec
/build/watchdog #

Attempting to fix the same by calling pm_runtime_disable.

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