HID: corsair: fix control-transfer error handling

Make sure to check for short control transfers in order to avoid parsing
uninitialised buffer data and leaking it to user space.

Note that the backlight and macro-mode buffer constraints are kept as
loose as possible in order to avoid any regressions should the current
buffer sizes be larger than necessary.

Fixes: 6f78193e

 ("HID: corsair: Add Corsair Vengeance K90 driver")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>