watchdog: omap: don't disable runtime pm before starting device

omap_wdt_start calls pm_runtime_get_sync so dropping a reference just
before calling omap_wdt_start doesn't make much sense. Moreover there is
no point to use the synchronous variant of pm_runtime_put because the
driver doesn't care if the clock is disabled before or after
omap_wdt_probe returns.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>