net: qualcomm: rmnet: Skip processing loopback packets

RX handler for RMNET driver was processing packets that were
looped back via dev_loopback_xmit() leading to strange behavior
like freeing fragments of valid SKBs being processed elsewhere
on the stack.

Change-Id: I9348a06a93c3e20d638dbba3a114e0e5b10c8ac4
CRs-fixed: 2283841
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>