dma: at_xdmac: change chunk size default value

Some devices don't set the maxburst value, in this case it is safer to
use a chunk of 1 instead of 16. For example, atmel_serial doesn't
set maxburst and having a chunk of 16 causes receiving 4 characters
intstead of 1.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>