alx: fix MAC address alignment problem

In two places, parts of MAC addresses are used as u32/u16
values. This can cause alignment problems, use put_unaligned
and get_unaligned to fix this.

Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>