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.

Kernel commit 3ff195b0, "sysfs: Implement sysfs tagged
directory support" patch, added a filtering mechanism
to sysfs, allowing sysfs directories to look different
depending on the context where they are being observed.

When an interface is moved to another namespace, the
interface dissapears from sysfs structure. In order
to recover access to the directory a solution is to
remount sysfs from the correct context. This will force
the user to remount sysfs before using iw from a
different namespace.

To avoid this issue 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
the device index.

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