Some host controllers will not operate without a hardware
timeout that is limited in value.  However large discards
require large timeouts, so there needs to be a way to
specify the maximum discard size.

A host controller driver may now specify the maximum discard
timeout possible so that max_discard_sectors can be calculated.

However, for eMMC when the High Capacity Erase Group Size
is not in use, the timeout calculation depends on clock
rate which may change.  For that case Preferred Erase Size
is used instead.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>