dma: debug: no need to check return value of debugfs_create functions

When calling debugfs functions, there is no need to ever check the
return value.  The function can work or not, but the code logic should
never do something different based on this.

Also delete the variables for the file dentries for the debugfs entries
as they are never used at all once they are created.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
[hch: moved dma_debug_dent to function scope and renamed it]
Signed-off-by: Christoph Hellwig <hch@lst.de>