octeontx2-af: Fix an off by one in rvu_dbg_qsize_write()

This code does not allocate enough memory for the NUL terminator so it
ends up putting it one character beyond the end of the buffer.

Fixes: 8756828a

 ("octeontx2-af: Add NPA aura and pool contexts to debugfs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>