ssb: extif: add methods for watchdog driver

The watchdog driver wants to set the watchdog timeout in ms and not in
ticks, add a method converting ms to ticks before setting the watchdog
register. Return the ticks or millisecond the timer was set to in case
the provided value was bigger than the max allowed value.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>