Currently when the call to prism2sta_ifst fails a netdev_err error
is reported, error return variable result is set to -1 but the
function always returns 0 for success.  Fix this by returning
the error value in variable result rather than 0.

Addresses-Coverity: ("Unused value")
Fixes: 00b3ed16 ("Staging: add wlan-ng prism2 usb driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200114181604.390235-1-colin.king@canonical.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>