mfd: cros_ec: spi: Calculate delay between transfers correctly

To avoid spamming the EC we calculate the time between the previous
transfer and the current transfer and force a delay if the time delta
is too small.

However, a small miscalculation causes the delay period to be
far too short. Most noticably this impacts commands with a long
turnaround time such as EC firmware reads and writes.

Signed-off-by: David Hendricks <dhendrix@chromium.org>
Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Andrew Bresticker <abrestic@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>