Several small fixes for the gpioths (temp/humidity sensor) driver.

At the end of a read cycle, set the gpio pin to INPUT rather than OUTPUT.
The state of the single-wire "bus" when idle should be high; setting the
pin to input allows the external pullup to pull the line high. Setting it
to output (and leaving it driving low) was leading a good read cycle followed
by one that would fail, and it just continued like that forever, effectively
reading the sensor once every 10 seconds instead of 5.

In the attach function, do an initial read from the device before registering
the sysctls for accessing the last-read values, to prevent reading spurious
values for the first 5 seconds after the driver attaches.

Do a callout_drain() in the detach function to prevent crashes after
unloading the module.