HID: avoid '\0' in hid debugfs events file

HID: avoid '\0' in hid debugfs events file

When dumping /sys/kernel/debug/hid/$device/events '\0' characters show up
(invisible if cat to console but shown by less or while looking at a dump
 file).  These are due to hid_debug_event() adding strlen()+1 bytes to the ring
buffer (e.g. including the trailing '\0').  Any roll-over causes a '\0' as well
as hid_debug_event() handles the ring buffers with HID_DEBUG_BUFSIZE-1 size
while hid_debug_events_read() handles it with full HID_DEBUG_BUFSIZE size.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>