mmc: atmel-mci: correct data timeout computation

The HSMCI operates at a rate of up to Master Clock divided by two.
Moreover previous calculation can cause overflows and so wrong
timeouts.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>