iio:light:stk3310: add more error handling

Check for the following error cases:
  * lower boundary for val in _write_event
  * return value of regmap_(field_)read
  * possible values for chan->type
  * return value of stk3310_gpio_probe

Also add an error path in _probe to put the sensor back into stand-by mode
in case of serious errors.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>