irqchip/loongson-pch-ms: Use bitmap_free() to free bitmap

kfree() and bitmap_free() are the same. But using the latter is more
consistent when freeing memory allocated with bitmap_zalloc().

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/0b982ab54844803049c217b2899baa59602faacd.1640529916.git.christophe.jaillet@wanadoo.fr