gpiolib: cdev: Ignore reconfiguration without direction

linereq_set_config() behaves badly when direction is not set.
The configuration validation is borrowed from linereq_create(), where,
to verify the intent of the user, the direction must be set to in order to
effect a change to the electrical configuration of a line. But, when
applied to reconfiguration, that validation does not allow for the unset
direction case, making it possible to clear flags set previously without
specifying the line direction.

Adding to the inconsistency, those changes are not immediately applied by
linereq_set_config(), but will take effect when the line value is next get
or set.

For example, by requesting a configuration with no flags set, an output
line with GPIO_V2_LINE_FLAG_ACTIVE_LOW and GPIO_V2_LINE_FLAG_OPEN_DRAIN
set could have those flags cleared, inverting the sense of the line and
changing the line drive to push-pull on the next line value set.

Skip the reconfiguration of lines for which the direction is not set, and
only reconfigure the lines for which direction is set.

Fixes: a54756cb

 ("gpiolib: cdev: support GPIO_V2_LINE_SET_CONFIG_IOCTL")
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20240626052925.174272-3-warthog618@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>