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-gd2980d8d 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>

tools/testing/selftests/gpio/gpio-sim.sh

@@ -389,6 +389,9 @@ create_chip chip
 create_bank chip bank
 set_num_lines chip bank 8
 enable_chip chip
+DEVNAME=`configfs_dev_name chip`
+CHIPNAME=`configfs_chip_name chip bank`
+SYSFS_PATH="/sys/devices/platform/$DEVNAME/$CHIPNAME/sim_gpio0/value"
 $BASE_DIR/gpio-mockup-cdev -b pull-up /dev/`configfs_chip_name chip bank` 0
 test `cat $SYSFS_PATH` = "1" || fail "bias setting does not work"
 remove_chip chip