mmc: core: Calculate the discard arg only once

In MMC, the discard arg is a read-only ext_csd parameter - set it once
on card init. To be consistent, do that for SD as well even though its
discard arg is always 0x0.

Signed-off-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

include/linux/mmc/card.h

@@ -277,6 +277,7 @@ struct mmc_card {
  	unsigned int		erase_shift;	/* if erase unit is power 2 */
  	unsigned int		pref_erase;	/* in sectors */
 	unsigned int		eg_boundary;	/* don't cross erase-group boundaries */
+	unsigned int		erase_arg;	/* erase / trim / discard */
  	u8			erased_byte;	/* value of erased bytes */
 
 	u32			raw_cid[4];	/* raw card CID */