iwlwifi: set MAC address in RXON from interface

When we do not have an interface, priv->mac_addr
is all zeroes, so the memcpy() is not useful as
the RXON buffer has been cleared previously.
Therefore, use the interface's address that we
are setting up the RXON for, if available.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>