cfg80211: Fix power save state after interface type change

Currently cfg80211 only configures the PSM state to the driver upon creation
of a new virtual interface, but not after interface type change. The mac80211
on the other hand reinitializes its sdata structure every time the interface
type is changed, losing the PSM configuration.

Hence, if the interface type is changed to, say, ad-hoc and then back to
managed, "iw wlan0 get power_save" will claim that PSM is enabled, when in
fact on mac80211 level it is not.

Fix this in cfg80211 by configuring the PSM state to the driver each time
the interface is brought up instead of just when the interface is created.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>