drivers: intc: stm32: correct inconsistent parameter names

Correct several inconsistent parameter names in the following functions:
- stm32_gpio_intc_select_line_trigger: rename `trigger` to `trg`
  to match the header file.
- stm32_gpio_intc_set_irq_callback: rename the callback argument to `user`
  to match the `stm32_gpio_irq_cb_t` type.
- stm32_exti_get_line_src_port: rename `pin` to `line` to align with
  the Doxygen comment and implementation.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>