net: apps: Fix the 802.15.4 monitor application

The 802.15.4 monitor application had issues with connecting
pipes together and making sure the first byte from qemu is
not lost. Before starting real data connection we send five
null bytes and the discard them in receiving side.

Change-Id: I376ae4f79aaa1ea742640999ec5b3d9c57a1279a
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>