net: rmnet_data: Assign network_header instead of data in mac_header

Commit 1a37e412 ("net: Use 16bits for *_headers fields of
struct skbuff") changes the data type of mac_header from pointer to
__u16. As a result, it is invalid to assign data to mac_header
in architectures where NET_SKBUFF_DATA_USES_OFFSET is 0.

Change-Id: I97ce04e3747983839d3908ca8111fd588c8e43a2
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>