msm: ipa: enable GSI interrupt moderation

Enable 0.5ms and 20 packets interrupt moderation in GSI
for WAN downlink pipe. To make sure latecy does not change
aggregation timer is reduced to 0.5ms from 1ms.

CRs-Fixed: 2202013
Change-Id: I567082f44d6842bb090133cca72a1db6d8921fc1
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>