HID: Fix uninitialized variable "size" in hid-wiimote-debug

This variable is initialized conditionally, based on whether a wiimote
call succeeds.  However, the logic is not obvious to the compiler so it
throws a warning.  Eliminate the warning by initializing "size" to 0.

The warning is:
files/drivers/hid/hid-wiimote-debug.c:69:18: warning: 'size' may be used
uninitialized in this function [-Wmaybe-uninitialized]

Signed-off-by: Simon Que <sque@chromium.org>
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>