drivers: rmnet_perf: Validate fragment checksum

Fragments were being flushed to stack without performing QMAPv4 checksum
offload calculation. This adds the missing callback to the core RmNet
driver to perform this validation and allow its statistics to be updated
accordingly.

Change-Id: If1180b583d988192915ec503b0bd5c81cde50576
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>