net: bcmgenet: Use get_unligned_beXX() and put_unaligned_beXX()

It's convenient to use get_unligned_beXX() and put_unaligned_beXX() helpers
to get or set MAC instead of open-coded variants.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>