HID: usbhid: make usbhid_set_leds() static

usbhid_set_leds() is only used inside of usbhid/hid-core.c so no need to
export it.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>