ARM: MXC: mxcmmc: misc cleanups

Be more verbose on error messages and add one debug message.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Volker Ernst <volker.ernst@txtr.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Michał Mirosław <mirqus@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>