watchdog: da9055: Don't update wdt_dev->timeout in da9055_wdt_set_timeout error path

Otherwise, WDIOC_GETTIMEOUT returns wrong value if set_timeout fails.
This patch also removes unnecessary ret variable in da9055_wdt_ping function.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>