wireless: Print wiphy name in sysfs.

The index cannot be used to reliably reconstruct a phy
name, so explicitly add the phy name to sysfs so that scripts
can figure out the parent phy device for a particular
wireless interface.

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