dmaengine: at_xdmac: fix bad behavior in interleaved mode

When performing interleaved transfers with numf > 1, an extra line is
copied. The mbr.bc field is incremented once too often. The length of
the block is (BLEN+1) microblocks.

Signed-off-by: Sylvain ETIENNE <Sylvain.ETIENNE@ingenico.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Fixes: 4e538578 ("dmaengine: at_xdmac: handle numf > 1")
Signed-off-by: Vinod Koul <vinod.koul@intel.com>