mmc: mmc_test: Extend "Badly aligned" tests for 8-byte alignment

The "Badly aligned" tests, test reading/writing with alignments
of 1,2 and 3.  SDHCI now has 64-bit ADMA which has 8-byte
alignment, so extend the tests to test up to 7.

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