net: rmnet_data: Fix incorrect flags for rmnet_data devices

Userspace applications report that SIOCETHTOOL IOCTL fails for
option ETHTOOL_STXCSUM even though rmnet_data devices with
prefix were created. This is because commit I183ba7
("net: rmnet_data: adding support to GRO") replaced the dev
features with NETIF_F_GRO only.

Fix this this by specifying the expected set of dev features.

CRs-fixed: 860895
Change-Id: Ic0935718a3a3f7bab5ea70d81c7dff99ebf0a7fc
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>