ugpio_full_open: do not fail when direction and/or edge is not exported via sysfs

On some boards that don't export edge and/or direction file for pins in sysfs,
there is a bug in ugpio.c that makes ugpio_full_open fail in this case.
This patch should correct these issues.

Signed-off-by: Fredrik Svensson <fredrik.svensson@axis.com>