atmel_mci: Fix data timeout value

Calculate the data timeout based on values from the CSD instead of
just using a hardcoded DTOR value. This is a backport of a similar fix
in BSP 2.0, with one additional fix: the DTOCYC value is rounded up
instead of down.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>