mmc: mmci: use NSEC_PER_SEC macro

This patch replaces a constant used in calculating timeout with a proper
macro. This is make code more readable.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>