net: linkaddr: calculate linkaddr storage addr size via config

- Introduce NET_LINK_ADDR_MAX_LENGTH which is either 6 or 8
  depending on whether CONFIG_NET_L2_IEEE802154 is used
- Instead of being a placeholder single index array of uint8_t,
  let's use NET_LINK_ADDR_MAX_LENGTH to assign the size of the
  "addr" array field in the net_linkaddr_storage structure.
- Now that the "addr" field of net_linkaddr_storage contains the
  true size of the link address, we can remove "storage" field
  which was hard coded to 8 bytes (2 uint32_t's).
- Fix 2 references to the "storage" field of the net_linkaddr_storage
  structure.

Change-Id: I2ea12058280b289f65085964eb7d503d4fd260c2
Signed-off-by: Michael Scott <michael.scott@linaro.org>