brcm47xx: b44: add dummy phy device if we do not find any

On some devices with e.g. a BCM2535F switch the second MAC is used, but
we can not find a phy under all addresses between 0 and 31, 0xffffffff
is returned as phy id. If we can not find a phy at the expected address
create a dummy one.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38538 3c298f89-4303-0410-b956-a3cf2f4a3e73