irqchip/armada-370-xp: Refactor mpic_handle_msi_irq() code

irqchip/armada-370-xp: Refactor mpic_handle_msi_irq() code

Refactor the mpic_handle_msi_irq() function to make it simpler:
- drop the function arguments, they are not needed
- rename the variable holding the doorbell cause register to "cause"
- rename the iterating variable to "i"
- use for_each_set_bit() (requires retyping "cause" to unsigned long)

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/all/20240711115748.30268-10-kabel@kernel.org