msm: rndis_ipa: fix header length calculation

When deaggreagtion is enabled we shouldn't add the rndis
header length to the total length of the USB to IPA pipe.
Add the length of the rndis header only when aggregation is
disabled.

Change-Id: Id17d2f6e6a371db393c099853f7f96f4ec0008d2
Signed-off-by: Amir Levy <alevy@codeaurora.org>