mmc: atmel-mci: atmci_convert_chksize depends on controller version

The atmci_convert_chksize() function is no more valid for controller
version 0x600 due to the introduction of '2 data' chunk size.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>