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>