mmc: sdhci: Fix incorrect ADMA2 descriptor table size

The ADMA2 descriptor table size was being calculated incorrectly
Fix it.

Note that it has been wrong for a long time and likely has not
caused any problems because of a combination of 1) not needing
alignment descriptors for block operations 2) more memory being
allocated than was requested 3) the use of
SDHCI_QUIRK_NO_ENDATTR_IN_NOPDESC which does not use an extra
descriptor for the end marker.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>