net: qualcomm: rmnet: Set skb->protocol before DL checksum

rmnet_map_checksum_downlink_packet() requires the skb->protocol field
to be set properly to perform downlink checksum validation.

Ensure this field is set before invoking this API.

Change-Id: If61b930e4b28dbced44ded343bf73b4d25d2df13
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>