serial: stm32: fix the get_irq error case

Checks the returned values of platform_get_irq() for both required
"event" and optional "wakeup" IRQs during probe. This allows the driver
probe to be deferred if needed.
Removes redundant checks for 'cfg.has_wakeup'.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>