[U-Boot] usb, composite: after unregister gadget driver set composite to NULL

[Marek Vasut]

Dear Heiko Schocher,

> Without this, second usb_composite_register() call fails always
> with -EINVAL.
> 
> Signed-off-by: Heiko Schocher <hs at denx.de>
> Cc: Lukasz Majewski <l.majewski at samsung.com>
> Cc: Kyungmin Park <kyungmin.park at samsung.com>
> Cc: Marek Vasut <marex at denx.de>

Applied, thanks.

Best regards,
Marek Vasut