net: rmnet_data: Checksum offload handle IPv4 UDP frames with 0 checksum

Checksum offload routine should skip checksum fixup computation on
IPv4 UDP packets which have the checksum field set to 0 by the sender.
This is allowed by RFC768. Packets are marked as checksum unnecessary
and shipped up the stack as-is.

Change-Id: I0432c3e1b25196134ecc8bbbe23c9cab46666d5c
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>