rmnet_core: Manually checksum csum_valid = 0 packets

The csum_valid bit being set to zero in the v5 csum offload header has two
possible meanings. Either 1) the checksum is bad, or 2) the checksum was
not calculated. When a packet is received with such a header, we need to
manually checksum the packet to avoid reporting potentially valid packets
as having bad checksums.

Change-Id: I6a85d7a01c844be625c11c80ba381ac4dbd0366d
Signed-off-by: Sharath Chandra Vurukala <quic_sharathv@quicinc.com>