mmc: dw_mmc: fix timeout calculate method

There are two cases not been considered:
- use uint for timeout, it will overflow when size bigger than 512KB for
  it *8*1000 at the beginning, but we may use size up to 32MB; The
  'timeout' will overflow if size bigger than 51.2MB after this fix, which
  should be enough for U-Boot;
- The timeout is using clock speed for data rate, but the device may not
  have such high speed, eg. clock is 52MHz while the device write speed may
  be less than 10MB/s, and we may use up to 150MHz clock.

Fix them in this patch, the max timeout is about 6500 when size is 32MB
after fix.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>