backports: fix led_trigger warning with old kernel versions

backports: fix led_trigger warning with old kernel versions

This fixes the following warning:

/drivers/net/wireless/at76c50x-usb.c: In function ‘at76_mod_init’:
/drivers/net/wireless/at76c50x-usb.c:2472:2: warning: passing argument 2 of ‘led_trigger_register_simple’ from incompatible pointer type [enabled by default]
  led_trigger_register_simple("at76_usb-tx", &ledtrig_tx);
  ^
In file included from /backport-include/linux/leds.h:3:0,
                 from /drivers/net/wireless/at76c50x-usb.c:45:
include/linux/leds.h:97:13: note: expected ‘struct led_trigger **’ but argument is of type ‘struct backport_led_trigger **’
 extern void led_trigger_register_simple(const char *name,
             ^
/drivers/net/wireless/at76c50x-usb.c: In function ‘at76_mod_exit’:
/drivers/net/wireless/at76c50x-usb.c:2484:2: warning: passing argument 1 of ‘led_trigger_unregister_simple’ from incompatible pointer type [enabled by default]
  led_trigger_unregister_simple(ledtrig_tx);
  ^
In file included from /backport-include/linux/leds.h:3:0,
                 from /drivers/net/wireless/at76c50x-usb.c:45:
include/linux/leds.h:99:13: note: expected ‘struct led_trigger *’ but argument is of type ‘struct backport_led_trigger *’
 extern void led_trigger_unregister_simple(struct led_trigger *trigger);
             ^

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>