staging: mt7621-mmc: Remove error message in debug

If the debug file in proc fs is not successfully created current code
prints an error message, this is removed. de is also not an error
pointer, so the IS_ERR() call leads to a static checker warning. The
unused return value of the msdc_debug_proc_init function is also removed.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>