drivers: intc: stm32: add break to switch-clause

add an unconditional break to switch-clause's default case, complying
with required [misra-c2012-16.3] rule which states; An unconditional
break statement terminate every switch-clause.

Found as a coding guideline violation (Rule 16.3) by static code
scanning tool.

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

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