net/fec: cleanup types in fec_get_mac()

My static checker complains that on some arches unsigned longs can be 8
characters which is larger than the buffer is only 6 chars.
Additionally, Ben Hutchings points out that the buffer actually holds
big endian data and the buffer we are reading from is CPU endian.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>