dm: test: Fix partition test to use mmc2

d94d9844 ("dm: part: Update test to use mmc2") attempted to make the
test use mmc2, but the change was incomplete in that it didn't also
change the strings that reference a specific partition. Fix these so
that the test passes again

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>