rmnet_core: pass correct length to ip_fast_csum

ip_fast_csum() takes in the header length in 4-byte words, not the length
in bytes.

Change-Id: Ib3941876e21260aad32ed761427714e133220cfb
Signed-off-by: Sean Tranchetti <quic_stranche@quicinc.com>
Signed-off-by: Kaustubh Pandey <quic_kapandey@quicinc.com>