hwmon: (lis3) Fix Oops with NULL platform data

The recent addition of threaded irq handler causes a NULL dereference
when used with hp_accel driver, which has NULL pdata.

Acked-by: Samu Onkalo <samu.p.onkalo@nokia.com>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>