ieee802154: change wpan-phy name to phy

Currently the wpan_phy under /sys/class/ieee802154/ is named as
"wpan-phy#", this patch will change the name to phy. This will
introduce the same naming convention like wireless.

Note: wpan-tools users will not type "wpan-phy#" anymore, just a simple
      "phy#" is enough.

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