usb: gadget: don't leak configs when unbinding

By the time g_dnl_unbind() is run, cdev->config has been set to NULL,
so the free() there does nothing, and the config struct is leaked.
Equally, struct usb_gadget contains a linked list of config structs, so
the code should iterate over them all and free each one, rather than
freeing one particular config struct.

composite_unbind() already iterates over the list of config structs, and
unlinks each from the linked list. Fix this loop to free() each struct as
it's unlinked and otherwise forgotten.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>