net: qualcomm: rmnet: Workaround for HW checksum error

When processing a coalesced frame with only a single packet, the error
bitmask can be incorrect in certain circumstatnce. When the coalescing
frame is closed due to the hardware hitting the packet limit, the byte
limit, the time limit, or encountered an EOF during DMA, the checksum will
always be marked is valid.

In the case where we receive such a frame, we need to checksum the packet
ourselves to determine if the checksum is actually valid.

Change-Id: Ia14c627231731f5484609fd135549ed2848a88d4
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>