Staging: dgrp: Refactor the function dgrp_receive() in drrp_net_ops.c

The function dgrp_receive() in dgrp_net_ops.c is too long and can be
refactored. It uses various switch statements and goto labels. I have
removed a label called data and tried to extract a new function out of
it called as handle_data_in_packet().

This helps to make the code more modularize and simple to read and
understand.

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>