drivers: intc: stm32: clarify controlling expression

add explicit boolean type to 'if' statement controlling expression,
consolidating it with 'stm32_exti_is_pending' function return type, thus
improving code readability and maintainability, complying with required
[misra-c2012-14.4] rule which states; The controlling expression of an
if statement and the controlling expression of an iteration-statement
shall have essentially boolean type.

Found as a coding guideline violation (Rule 14.4) by static code
scanning tool.

Note: Tested on STM32L5 Nucleo-144 board (stm32l552xx).

Signed-off-by: ferar alashkar <ferar.alashkar@gmail.com>