USB: ipw: fix interface-data memory leak in error path

Move interface data allocation to attach so that it is deallocated
should usb-serial probe fail.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>