dma-debug: Cleanup for copy-loop in filter_write()

Earlier in this function we set the last byte of "buf" to NULL so we
always hit the break statement and "i" is never equal to NAME_MAX_LEN.
This patch doesn't change how the driver works but it silences a Smatch
warning and it makes it clearer that we don't write past the end of the
array.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>