net: qualcomm: rmnet: treat IPv4 UDP zero checksum as valid

UDPv4 zero checksum packets are marked as having an invalid checksum by
the hardware. This can cause problems when segmenting coalesced packets
with zero checksum, as RmNet will build a bad checksum for them, thinking
that the packets need to be dropped by the stack.

Change-Id: I4f5eb13fb6ae3664185d3f6703aaff978bcdfb4e
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>