tracing: Fix regression in printk_formats file

The fix to fix the printk_formats of modules broke the
printk_formats of trace_printks in the kernel.

The update of what to show via the seq_file was only updated
if the passed in fmt was NULL, which happens only on the first
iteration. The result was showing the first format every time
instead of iterating through the available formats.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>