gpio: selftests: gpio-mockup-chardev GPIOHANDLE_REQUEST_OUTPUT fix

Following logic from commit: 22f6592b

, GPIOHANDLE_REQUEST_OUTPUT
should handle errors same way as GPIOHANDLE_REQUEST_INPUT does, or else
the following error occurs:

gpio-mockup-chardev: gpio<gpiochip1> line<0> test flag<0x2> value<0>: No
such file or directory

despite the real result of gpio_pin_test(), gpio_debugfs_get() and
gpiotools_request_linehandle() functions.

Signed-off-by: Rafael David Tinoco <rafael.tinoco@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>