mmc: Register debugfs dir before calling card probe function.

This way, the probe function may register debugfs files if it wants to.
This fixes a bug with mmc_test where mmc_test_register_file_test() is
called before the card's debugfs dir exists, and so it fails.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Tested-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Chris Ball <cjb@laptop.org>