wireless: Use first phyX name available when registering phy devices.

Choose first available phyX name when creating phy devices.  This
means that reloading a wifi driver will not cause a change in the
name of it's phy device.

Also, allow users to rename a phy to any un-used name, including
phy%d.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>