Add the missing unlock before return from function ecm_alloc()
in the error handling case.

Introduced by commit da92801c.
(usb: gadget: f_ecm: add configfs support)
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>