docs: driver-api pinctrl cleanup

docs: driver-api pinctrl cleanup

Replace FIXME comments in the pinctrl documentation example with
proper cleanup code:
- Add devm_pinctrl_put() calls in error paths
  (pinctrl_lookup_state, pinctrl_select_state)
  after successful devm_pinctrl_get()
- Set foo->p to NULL when devm_pinctrl_get() fails
- Add ret variable for cleaner error handling
- provides proper example of pinctrl resource management on failure

Signed-off-by: Alex Tran <alex.t.tran@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250827074525.685863-1-alex.t.tran@gmail.com