watchdog: refactor watchdog_init_timeout

The function is not easy to read and has a problem: -EINVAL is returned
when the module parameter is invalid but the DT parameter is OK.

Refactor the code to have the same pattern of checks for the module
parameter and DT. Further ones can be easily added in the future if the
need arises. The above mentioned problem is fixed, too.

Some documentation is added to describe the different handlings of '0'
for the module parameter and the DT property.

Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>