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>

drivers/mmc/core/core.c

@@ -2369,7 +2369,7 @@ unsigned int mmc_calc_max_discard(struct mmc_card *card)
 		return card->pref_erase;
 
 	max_discard = mmc_do_calc_max_discard(card, MMC_ERASE_ARG);
-	if (mmc_can_trim(card)) {
+	if (max_discard && mmc_can_trim(card)) {
 		max_trim = mmc_do_calc_max_discard(card, MMC_TRIM_ARG);
 		if (max_trim < max_discard)
 			max_discard = max_trim;