gpio: mcp23s08: Do not free unrequested interrupt

If devm_request_threaded_irq fails for some reason we call
mcp23s08_irq_teardown afterwards.
Do not free the unrequested interrupt in this case. free_irq can also be
omitted for the error free case because we use devm_request_threaded_irq.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>