mmc: dw_mmc: use macro to define ring buffer size

It's very prone to make mistake as we might forget
to replace all PAGE_SIZEs with new values if we try
to modify the ring buffer size for whatever reasons.
Let's use a macro to define it.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>