drivers: watchdog: Pass timeout in ms if iwdg is started at boot

iwdg_stm32_install_timeout expects a timeout passed in
milli seconds. As the timeout is defined in micro
seconds in Kconfig.stm32, we need to divide by
USEC_PER_MSEC when CONFIG_IWDG_START_AT_BOOT is
activated because iwdg_stm32_install_timeout makes
the multiplication by USEC_PER_MSEC.

Fixes #18695

Signed-off-by: Philémon Jaermann <p.jaermann@gmail.com>