mmc: dw_mmc: set fixed burst in BMOD register

This patch uses the fixed burst bit when using an internal DMA controller.
I found increased performance with IDMAC when this bit is set.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>