iio:light:stk3310: move device register to end of probe

iio_device_register should be the last operation during probe. Therefor
move up interrupt setup code and while at it, change the check for invalid
values of client->irq to be smaller than zero.
Fixes: 3dd477ac ("iio: light: Add threshold interrupt support for STK3310")

As the device_register makes the userspace interfaces of the device available
it is possible for requests to come in before the probe sequence has finished.
This can lead to unhandled interrupts and similar.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Reviewed-by: Tiberiu Breana <tiberiu.a.breana@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>