net: prefer ETHER_ADDR_LEN over ETH_ADDR_LEN

A couple of drivers and one place in if.c use ETH_ADDR_LEN, even though
net/ethernet.h provides an equivalent ETHER_ADDR_LEN definition.

Cleanup all of the locations which refer to ETH_ADDR_LEN to use the
standard ETHER_ADDR_LEN instead.

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

Submitted by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed by: erj@, jpaetzel@
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D21239