Input: mxs-lradc - use correct error check

Function devm_ioremap() will return a NULL pointer on failure. However,
in function mxs_lradc_ts_probe(), its return value is checked with
IS_ERR(), which cannot detect the exceptional case.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>