mmc: Change the max discard sectors and erase response when HW busy detect

When mmc host HW supports busy signalling (using R1B as response), don't
use the host->max_busy_timeout as the limitation when deciding the max
discard sectors, which we inform the generic BLOCK layer about. Instead,
let's use at least one preferred erase size as the max discard sectors.

In cases when the host controller supports HW busy signalling and the
timeout for the erase operation doesn't exceed the max_busy_timeout, we
keep the R1B response, otherwise we prevent the host from doing HW busy
detection by converting to a R1 response.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>