mmc: debugfs: Print ext_csd in ascending order

ext_csd exported through debugfs is printed in reverse order (from
byte 511 to 0), which causes confusion.
Fix the for loop to print ext_csd in natural order.

Signed-off-by: Venkatraman S <svenkatr@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>