net: Check the return value when sending packet

If the return value of the sender is 0, then packet is/was
discarded and we release the net_buf. If the value is 1, then
packet was sent properly and the driver has already freed
the net_buf.

Change-Id: Icd93602225e14f7210fe9ef33603a7451d834a80
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>