usb: gadget: configfs: keep a function if it is not successfully added

If usb_add_function() fails then the currently processed function
is already not in the list in struct config_usb_cfg, and neither is it
in the list in struct usb_configuration. At the err_purge_funcs label the
purge_config_funcs() is called, which iterates over all configurations,
and in each configuration it iterates over all _successfully_ added
functions, and moves them back from the list in struct usb_configuration
to the list in struct config_usb_cfg. BUT the function which has just
failed adding and caused the unwind process is not taken care of and
is effectively lost.

This patch modifies the configfs_composite_bind() function so that if
the usb_add_function() fails, then the currently processed function
is returned to the list in struct config_usb_cfg.

It would be tempting to delay the list_del() in question after
usb_add_function() invocation, but a struct list_head (&f->list) cannot be
stored in more than one list at the same time, so the list_del() must
be called before usb_add_function(). Hence, the solution is to list_add()
after usb_add_function() in case of error.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>