net: Additional checks for length of the data to be sent

Dummy 802.15.4 has now some additional check to verify that we
are really sending correct amount of bytes.

Change-Id: I4d223c02e2628966e3b1a3cdcf7b560043d7d788
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>