net: add ETHER_IS_ZERO macro similar to ETHER_IS_BROADCAST

Some places in network code may need to verify that an ethernet address
is not the 'zero' address. Provide a standard macro ETHER_IS_ZERO for
this purpose, similar to the ETHER_IS_BROADCAST macro already available.

This patch also removes previous ETHER_IS_ZERO definitions in several
USB ethernet drivers, in favor of this centrally-located macro.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>

Submitted by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed by: erj@
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D21240