USB: ehci-dbg: fix reading less content of periodic file

This patch fix 2 problems about reading periodic file:

1. The "..." after a interrupt qh is missed because buffer pointer is
   not moved.

2. After setting p.ptr as NULL, its next qh or itd will be omited and
   can't be stored in debug buffer.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>