drivers: watchdog: iwdg_stm32: fix wdg install timeout

In the function iwdg_stm32_install_timeout, the test on watchdog ready
was inverted. So, if 2 successive calls were made to this function, the
value of the prescaler or counter reload was not taken into account.

Signed-off-by: Julien D'Ascenzio <julien.dascenzio@paratronic.fr>