USB: ehci-dbg: increase debug buffer size for periodic file

This patch is based on the following ideas:

1. Some usb devices (such as usb video class) have endpoints of high
   interval attribute, so reading "periodic" file need more debug buffer
   to accommodate the qh or itd schedule information.  For example, 4KB
   buffer is not enough for a single interrupt qh of 2ms period.

2. print a %p need 16 byte buffer on 64-bits arch, but 8 byte on 32-bits
   arch. Add a extra bonus for 64-bits arch.

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>