HID: hid-sensor-custom: Fix big on-stack allocation in hid_sensor_custom_get_known()

struct hid_sensor_custom_properties is currently 384 bytes big, which consumes
too much stack space for no good reason. Make it dynamically allocated.

Fixes: 98c062e8 ("HID: hid-sensor-custom: Allow more custom iio sensors")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>