selftests: gpio: fix uninitialised variable warning

When compiled with -Wall gpio-mockup-cdev.c reports an uninitialised
variable warning.  This is a false positive, as the variable is ignored
in the case it is uninitialised, but initialise the variable anyway
to remove the warning.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>