usb: gadget: tcm_usb_gadge: fix to return error or 0 in tcm_usbg_drop_nexus()

In the error handling case of tcm_usbg_drop_nexus(), the error code
is assigned to 'ret', but it is ignored. We'd better return 'ret'
instead of always return 0.

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Felipe Balbi <balbi@ti.com>