mmc: dw_mmc: fix the wrong max_blk_size

According to DesignWare TRM, BLKSIZ is 16bits.
Then it's correct that max_blk_size should be 0xFFFF, not 0x10000.

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