mmc: mmc_test: align max_seg_size

If max_seg_size is unaligned, mmc_test_map_sg() may create sg element
sizes that are not aligned with 512 byte. Fix, align max_seg_size at
mmc_test_area_init().

Signed-off-by: Per Forlin <per.forlin@stericsson.com>
Signed-off-by: Chris Ball <cjb@laptop.org>