HID: roccat: Add ioctl command to retreive report size from chardev

Roccat chardev was reworked to support only a defined report size per
device and this can be retreived by an ioctl now to enable future changes
in report definitions.
Header was moved/renamed from drivers/hid to include/linux for accessibility.

Signed-off-by: Stefan Achatz <erazor_de@users.sourceforge.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>