device property: Add ETH_ALEN check, update comments.

This patch adds MAC address length check back into
the device_get_mac_addr() function before calling
is_valid_ether_addr() similar to the way the OF
routine does it.

Update the comments for the two new functions.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>