drivers: rmnet_perf: Set payload length for all packets

For non-TCP/UDP and fragmented packets, the payload length is not set in
the pkt_info struct. This can cause an invalid checksum offload trailer to
be passed to the core driver when validating checksum packets.

157.995781: <2> rmnet_map_checksum_downlink_packet+0x68/0x37c
158.001428: <2> rmnet_perf_core_validate_pkt_csum+0x94/0xc4
158.008053: <2> rmnet_perf_core_handle_packet_ingress+0x60/0x9c
158.015031: <2> __rmnet_perf_core_deaggregate+0x274/0x2d0
158.021475: <2> rmnet_perf_core_deaggregate+0x64/0x144
158.027646: <2> rmnet_rx_handler+0x150/0x1c8

Change-Id: I1d565e214469f00e4892ce487ee750b1427f0f58
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>