mmc: dw_mmc: fix the max_blk_count in IDMAC

Even though 1MB is reserved for descriptor table in IDMAC,
the dw_mmc host driver is allowed to receive only maximum
128KB block length in one request. This is caused by setting
improper max_blk_count. It needs to be e adjusted so that
descriptor table is used fully. It is found that the performance
is improved with the increased the max_blk_count.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>