staging: vt6656: iwctl: remove redundant cast (to PSDevice)

netdev_priv() returns a void*, so there is no reason to explicitly
cast to (PSDevice) when assigning to a variable of type PSDevice. The
cast is done implicitly.
Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>