etherdevice.h: Add ether_addr_equal

Add a boolean function to check if 2 ethernet addresses
are the same.

This is to avoid any confusion about compare_ether_addr
returning an unsigned, and not being able to use the
compare_ether_addr function for sorting ala memcmp.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>