cfg80211: fix race between sysfs and cfg80211

device_add() is called before adding the phy to the cfg80211 device
list.

So if a userspace program uses sysfs uevents to detect new phy
devices, and queries nl80211 to get phy info, it can get ENODEV even
though the phy exists in sysfs.

An easy workaround is to hold the cfg80211 mutex until the phy is
present in sysfs/cfg80211/debugfs.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>