mmc: omap: Use DIV_ROUND_UP instead of open coded

Also uses NSEC_PER_SEC and USEC_PER_SEC instead of hard-coded value.
This makes the intention more clear.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>