selftests: watchdog: prefer strtoul() over atoi()

commit f15d7114 ("Documentation/watchdog: add timeout and ping rate
control to watchdog-test.c") used both atoi() and strtoul() for string
to integer conversion. As usage of atoi() is discouraged in newer code,
replace it with strtoul() for consistency.

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>