drivers: rmnet_perf: Fix pre-segmented descriptor handling

If an incoming descriptor was segmented for any reason by the core driver
during RSB/RSC handling (i.e. different sizes, checksum issues, or HW GRO
bit has been toggled off), rmnet_perf would handle the descriptor
improperly while looking for headers and calculating lengths. rmnet_perf
needs to check the frag_desc->hdr_ptr value while grabbing header info.

Change-Id: I3695081a03cc992aa2bc3775370668ecc0c9c3ab
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>