staging: wlan-ng: Fix problem with wrong arguments

The function pointer scan in struct cfg80211_ops is not
supposed to be assigned a function with a struct net_device
pointer as an argument. Instead access the net_device struct
in the following way:

struct net_device *dev = request->wdev->netdev;

sparse gives these warnings:

drivers/staging/wlan-ng/cfg80211.c:726:17: warning:
        incorrect type in initializer (incompatible argument 2
        (different base types))
        expected int ( *scan )( ... )
        got int ( extern [toplevel] *<noident> )( ... )

drivers/staging/wlan-ng/cfg80211.c:726:2: warning:
        initialization from incompatible pointer type [enabled by default]

drivers/staging/wlan-ng/cfg80211.c:726:2: warning:
        (near initialization for ‘prism2_usb_cfg_ops.scan’)
        [enabled by default]

Signed-off-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>