irqchip/gic-v3-its: Remove the redundant set_bit for lpi_map

We try to find a free LPI region in device's lpi_map and allocate them
(set them to 1) when we want to allocate LPIs for this device. This is
what bitmap_find_free_region() has done for us. The following set_bit
is redundant and a bit confusing (since we only set_bit against the first
allocated LPI idx). Remove it, and make the set_bit explicit by comment.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>