Set MMC_DATA_MULTI flag when doing multi-block transfers

Set MMC_DATA_MULTI flag when doing multi-block transfers

Lower layers (MMC / SDHCI controller drivers) may make certain decisions
based on the presence of this flag. The fact that sdhci.c doesn't
look at this flag is another problem that should be fixed separately.

Found when adding MMCCAM support to AllWinner MMC controller driver
where the presence of this flag actually matters.

Approved by:	imp (mentor)
Differential Revision:	https://reviews.freebsd.org/D15888