iw: use nl80211 for phy_lookup function

Original implementation uses sysfs to get dev index from
dev name. Due the changes on netns and sysfs iw is broken
if using multiple network namespaces. iw works properly
if using it from the main namespace, but it won't work if
using from the new namespace.

To avoid this issue with sysfs we can use nl80211
(using NL80211_CMD_GET_WIPHY command) this returns the
list of phys, then process the list, find the device and
return device index.

Signed-off-by: Javier Lopez <jlopex@cozybit.com>