HID: hid-input: fix compile for !HID_BATTERY_STRENGTH

As reported by Stephen Rothwell:
drivers/hid/hid-input.c: In function 'hidinput_hid_event':
drivers/hid/hid-input.c:865:6: error: 'struct hid_device' has no member
named 'battery_val'
drivers/hid/hid-input.c:866:3: error: 'struct hid_device' has no member
named 'battery_min'
drivers/hid/hid-input.c:866:3: error: 'struct hid_device' has no member
named 'battery_max'

Signed-off-by: Jeremy Fitzhardinge <jeremy@goop.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-input.c

@@ -861,13 +861,14 @@ void hidinput_hid_event(struct hid_device *hid, struct hid_field *field, struct
 
 	input = field->hidinput->input;
 
+#ifdef CONFIG_HID_BATTERY_STRENGTH
 	if (usage->hid == HID_DC_BATTERYSTRENGTH) {
 		hid->battery_val = value;
 		hid_dbg(hid, "battery value is %d (range %d-%d)\n",
 			value, hid->battery_min, hid->battery_max);
 		return;
 	}
-
+#endif
 	if (!usage->type)
 		return;