[media] coda: fix double call to debugfs_remove

In coda_free_aux_buf() call debugfs_remove only if buffer entry
is valid (and therfore dentry is valid), double protect by
invalidating dentry value.

Fixes erroneous prematurely dealloc of debugfs caused by
incorrect reference count incrementing.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>