net: Adapt return value from network driver send routine

This patch fixes a potential issue in which the signed return value
from the network driver send routine is returned from a caller that
has an unsigned return type.  The meaning of a negative return value
from the network driver send routine is that an error occurred.  A
return value of 1 means that the packet was sent successfully.  A
return value of 0 means that the packet could not be sent.  Thus, this
patch converts negative return values from the network driver send
routine to a return value of 0 from the caller.

Change-Id: If5cbecb18e514fd976200ecc45782d2a9e1f300f
Signed-off-by: Michael LeMay <michael.lemay@intel.com>