mmc: fixes for eMMC v4.5 discard operation

eMMC v4.5 discard operation is significantly different from the
existing trim operation because it is not guaranteed to work with
the new sanitize operation.  Consequently mmc_can_trim() is
separated from mmc_can_discard().

Also the new discard operation does not result in the sectors being
set to all-zeros, so discard_zeroes_data must not be set.

In addition, the new discard has the same timeout as trim, but from
v4.5 trim is defined to use the hc timeout.  The timeout calculation
is adjusted accordingly.

Fixes apply to linux 3.2 on.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: <stable@vger.kernel.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>