fbdev: mbx: fix up debugfs file creation

There is no need to keep the dentries around for the individual debugfs
files, just delete the whole directory all at once at shutdown instead.

This also fixes a tiny memory leak where the memory for the pointers to
the file dentries was never freed when the device shut down, as well as
making the logic of the code a lot simpler.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>