rmnet: Control DL csum offload with RXCSUM

Similar to the UL direction, allow the hardware checksum offload support
to be toggled off with the NETIF_F_RXCSUM flag though ethtool.

Change-Id: I38e43cf9c13363eee340793878be7639f18254e3
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>