drivers/watchdog: wwdg_stm32: Check return value of clock_control_on

In function wwdg_stm32_init, return value of clock_control_on
was not checked.
This is reported as an issue by coverity (CID 219600).
Fix this.

Fixes #33067

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>