HID: core: Fix size as type u32

When size is negative, calling memset will make segment fault.
Declare the size as type u32 to keep memset safe.

size in struct hid_report is unsigned, fix return type of
hid_report_len to u32.

Cc: stable@vger.kernel.org
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>