usb: gadget: composite: prevent a memory leak when configuration bind fails

In some USB composite gadget drivers, the configuration's bind function called
by the usb_add_config() calls multiple bind config functions. (for example cdc2
configuration bind function in the cdc_do_config() of the cdc2.c has two
functionality bind config functions.
  - the ecm_bind_config() & the acm_bind_config())

In each functionality bind config function, new instance is allocated and
finally added by the usb_add_function().

So if an error occurred during the second functionality bind config (for
example an error occurred at the acm_bind_config() after succeeding of the
ecm_bind_function()), the instance created by the acm_bind_config() cannot be
freed creating a memory leak.

This patch fixes this issue.

Signed-off-by: Yongsul Oh <yongsul96.oh@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>