mmc: core: optimize mmc_calc_max_discard

If the max_discard value is zero, the conditional branch that checks the
trim capabilities will never update this value with max_trim.

Change the condition statement to also check the max_discard value in order
to avoid an unnecessary call to mmc_do_calc_max_discard.

Signed-off-by: Sergio Valverde <vlvrdv@gmail.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chip.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>