msm: emac: Set checksum flag properly for received packets

When the receive checksum offload is enabled, set the checksum
flag for received packets indicating that no further checksum
validation is necessary.

Change-Id: Iea0b57dd951d979313bb8cf8c815a8cae74fee66
Signed-off-by: Niranjana Vishwanathapura <nvishwan@codeaurora.org>