HID: sony: Use the minimum accepted size for feature report 0xf2

Sixaxis devices accept feature report 0xf2 when size is >= 17, not 18.
Use the minimum accepted size.

The change is mainly for documentation purposes, the code worked fine
even before this change.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Acked-by: Frank Praznik <frank.praznik@oh.rr.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>