drivers: intc: stm32: correct for counter type

change for loop iterator/counter type to unsigned int, thus matching
that of struct stm32_exti_range's len member var, complying with
required [misra-c2012-10.4] rule which states; Both operands of an
operator in which the usual arithmetic conversions are performed shall
have the same essential type category.

Found as a coding guideline violation (Rule 10.4) by static code
scanning tool.

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

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