serial: at91: Use dmaengine_slave_config API

We are removing the dmaengine_device_control API, that shouldn't even have been
exposed in the first place. Change the callers to use the proper API.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>