etherdevice.h: Add ether_addr_equal_64bits

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

This is to avoid any confusion about compare_ether_addr_64bits
returning an unsigned, and not being able to use the
compare_ether_addr_64bits function for sorting ala memcmp.

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