irqchip/gic-v3-its: Simplify LPI_PENDBASE_SZ usage

LPI_PENDING_SZ is always used in conjunction with a max(), which doesn't
make much sense, since we're guaranteed that LPI_PENDING_SZ is already
aligned to 64K. Let's remove it.

Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Tested-by: Bhupesh Sharma <bhsharma@redhat.com>
Tested-by: Lei Zhang <zhang.lei@jp.fujitsu.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>