watchdog: sunxi: Readjust timeout on ioctl

SETTIMEOUT ioctl only adjusted the local timeout value but did not apply
it, so we were not setting the new timeout on the hardware at all until
the next watchdog stop and start cycle (which might not occur at all)

Fix it by running sunxi_wdt_set_timeout to readjust the hardware after
changing the local value.

Signed-off-by: Emilio López <turl@linux-sunxi.org>