selftests: gpio: gpio-sim: Fix BUG: test FAILED due to recent change

According to Mirsad the gpio-sim.sh test appears to FAIL in a wrong way
due to missing initialisation of shell variables:

 4.2. Bias settings work correctly
 cat: /sys/devices/platform/gpio-sim.0/gpiochip18/sim_gpio0/value: No such file or directory
 ./gpio-sim.sh: line 393: test: =: unary operator expected
 bias setting does not work
 GPIO gpio-sim test FAIL

After this change the test passed:

 4.2. Bias settings work correctly
 GPIO gpio-sim test PASS

His testing environment is AlmaLinux 8.7 on Lenovo desktop box with
the latest Linux kernel based on v6.2:

  Linux 6.2.0-mglru-kmlk-andy-09238-gd2980d8d8265 x86_64

Suggested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>