wireless: ipw2x00: convert to use DRIVER_ATTR_RW

We are trying to get rid of DRIVER_ATTR(), and the ipw2x00 driver's
attributes can be trivially changed to use DRIVER_ATTR_RW().

Cc: Stanislav Yakovlev <stas.yakovlev@gmail.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: <linux-wireless@vger.kernel.org>
Cc: <netdev@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>