mac802154: iface: check concurrent ifaces

This patch adds a check for concurrent interfaces while calling
interface up. This avoids to have different mac parameters on one phy.
Otherwise it could be that a interface can overwrite current phy mac
settings which is set by an another interface.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>