usb, composite: after unregister gadget driver set composite to NULL

Without this, second usb_composite_register() call fails always
with -EINVAL.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>