usb: gadget: f_ecm: Always set current gadget in ecm_bind()

usb: gadget: f_ecm: Always set current gadget in ecm_bind()

The gadget may change over bind/unbind cycles, so set it each time during
bind, not only the first time. Without it we get a use-after-free with
the following example:

cd /sys/kernel/config/usb_gadget/; mkdir -p mygadget; cd mygadget
mkdir -p configs/c.1/strings/0x409
echo "C1:Composite Device" > configs/c.1/strings/0x409/configuration
mkdir -p functions/ecm.usb0
ln -s functions/ecm.usb0 configs/c.1/
rmmod dummy_hcd
modprobe dummy_hcd

KASAN will complain shortly after the 'modprobe':

usb 2-1: New USB device found, idVendor=0000, idProduct=0000, bcdDevice= 6.01
usb 2-1: New USB device strings: Mfr=0, Product=0, SerialNumber=0
==================================================================
BUG: KASAN: use-after-free in gether_connect+0xb8/0x30c
Read of size 4 at addr cbef170c by task swapper/3/0

CPU: 3 PID: 0 Comm: swapper/3 Not tainted 6.1.0-rc3-00014-g41ff012f50cb-dirty #322
Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
 unwind_backtrace from show_stack+0x10/0x14
 show_stack from dump_stack_lvl+0x58/0x70
 dump_stack_lvl from print_report+0x134/0x4d4
 print_report from kasan_report+0x78/0x10c
 kasan_report from gether_connect+0xb8/0x30c
 gether_connect from ecm_set_alt+0x124/0x254
 ecm_set_alt from composite_setup+0xb98/0x2b18
 composite_setup from configfs_composite_setup+0x80/0x98
 configfs_composite_setup from dummy_timer+0x8f0/0x14a0 [dummy_hcd]
 ...

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20221104131031.850850-3-s.hauer@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>