USB: usbmon: mask seconds properly in text API

The code does not implement the comment, so timestamps for long traces
become confusing instead of wrapping neatly as expected. This was actually
observed. Fortunately for API being in debugfs, we can just fix this instead
of staying bug-for-bug compatible. Double fortunately, the stable binary
API is not affected.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>