msm: ipa: Fix to update the descriptor length in SG scenario

When sending a SG packet, frag descriptor length is not updated
and as a result IPA HW will stall. Make a change to update
descriptor length to the size of fragment.

Change-Id: Ie742148fe3b04677365a7247a89c2bed412e4d31
Acked-by: Chaitanya Pratapa <cpratapa@qti.qualcomm.com>
Signed-off-by: Sridhar Ancha <sancha@codeaurora.org>