usb: gadget: multi: fix error return code in cdc_do_config()

Fix to return a negative error code from the error handling
case instead of 0, as returned elsewhere in this function.

Introduced by commit 59835a (usb: gadget: multi: use
function framework for ACM.)

Make rndis_do_config() consistent with cdc_do_config() in the way it
handles returning the PTR_ERR(f_acm_*).

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>