dmaengine: at_xdmac: adapt perid for mem2mem operations

The PERID in the CC register for mem2mem operations must match an unused
PERID.
The PERID field is 7 bits, but the selected value is 0x3f.
On later products we can have more reserved PERIDs for actual peripherals,
thus this needs to be increased to maximum size.
Changing the value to 0x7f, which is the maximum for 7 bits field.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20201016093725.289880-1-eugen.hristev@microchip.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>