usb: r8a66597-udc: fix spinlock usage

Because the disconnect function in the composite driver will call spin_lock,
this driver has to call spin_unlock before calling driver->disconnet().

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>