env: mmc: Make the debug messages play a little nicer

Since we have global messages to indicate what's going on, the custom
messages in the environment drivers only make the output less readable.

Make MMC play a little nicer by removing all the extra \n and formatting
that is redundant with the global output.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>