mmc: core: Use a default maximum erase timeout

In cases when the host->max_busy_timeout isn't specified, the calculated
number of maximum discard sectors defaults to UINT_MAX. This may cause a
too long timeout for a discard request.

Avoid this by using a default maximum erase timeout of 60s, used when we
calculate the maximum number of sectors that are allowed to be discarded
per request.

Do note that the minimum number of sectors to be discarded is still at
least one "preferred erase size".

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