dmaengine: at_xdmac: rework slave configuration part

dmaengine: at_xdmac: rework slave configuration part

commit 765c37d876698268eea8b820081ac8fc9d0fc8bc upstream.

Rework slave configuration part in order to more report wrong errors
about the configuration.
Only maxburst and addr width values are checked when doing the slave
configuration. The validity of the channel configuration is done at
prepare time.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>