usb: gadget: f_subset: fix missing unlock on error in geth_alloc()

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

Introduced by commit 02832e56.
(usb: gadget: f_subset: 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>