irqchip/gic-v3-its: Fix comparison logic in lpi_range_cmp

The lpi_range_list is supposed to be sorted in ascending order of
->base_id (at least if the range merging is to work), but the current
comparison function returns a positive value if rb->base_id >
ra->base_id, which means that list_sort() will put A after B in that
case - and vice versa, of course.

Fixes: 880cb3cd (irqchip/gic-v3-its: Refactor LPI allocator)
Cc: stable@vger.kernel.org (v4.19+)
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>