mmc: sdhci: Correct ADMA_DESC_LEN to 12

Commit 37cb626d ("mmc: sdhci: Add Support for ADMA2") introduced
ADMA_DESC_LEN == 16 (64 bit case), but it was never used before commit
74755c1f ("mmc: sdhci: introduce adma_write_desc() hook to struct sdhci_ops").

"sizeof(struct sdhci_adma_desc)" (== 12 for 64bit case) was used instead.

Confusion probably originates from Linux commit 685e444bbaa0
("mmc: sdhci: Add ADMA2 64-bit addressing support for V4 mode"), but
the latter "V4 mode" was never ported to U-Boot.

Fixes: 74755c1f ("mmc: sdhci: introduce adma_write_desc() hook to struct sdhci_ops")
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Tested-by: Jonathan Humphreys <j-humphreys@ti.com>
Tested-by: Judith Mendez <jm@ti.com>