HID: core: zero-initialize the report buffer

Since the report buffer is used by all kinds of drivers in various ways, let's
zero-initialize it during allocation to make sure that it can't be ever used
to leak kernel memory via specially-crafted report.

Fixes: 27ce4050 ("HID: fix data access in implement()")
Reported-by: Benoît Sevens <bsevens@google.com>
Acked-by: Benjamin Tissoires <bentiss@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.com>