mmc_test: fix performance tests that go over max_blk_count

The host controller driver limits I/O transfers to maximum
transfer size, maximum block count, maximum segment size
and maximum segment count.  The performance tests were
not obeying these limits which meant they would not work
with some drivers.  This patch fixes that.
Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>