Fixes the following sparse warning:

drivers/input/touchscreen/mxs-lradc-ts.c:33:12: warning:
 symbol 'mxs_lradc_ts_irq_names' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
[dtor: changed to static const char * const]
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>