irqchip/stm32-exti: Read event trigger type from event_trg register

The flag reporting whether an event is 'direct' or 'configurable'
is available in the read-only registers EVENT_TRG.

Drop this redundant information from the struct stm32_desc_irq and
use the proper bit from EVENT_TRG register.
On armv7a this patch reduces by 3% the size of the driver, from
   text    data     bss     dec     hex filename
   7233     424       4    7661    1ded irq-stm32-exti.o
to
   6977     424       4    7405    1ced irq-stm32-exti.o
Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220606162757.415354-6-antonio.borneo@foss.st.com