stepper: shell: fix uninitialized variable compiler warning

Fixes:

/__w/zephyr/zephyr/include/zephyr/drivers/stepper.h:429:16: error:
'direction' may be used uninitialized [-Werror=maybe-uninitialized]

found with:

west build -p -b esp32s3_touch_lcd_1_28/esp32s3/appcpu -T
tests/drivers/stepper/shell/drivers.stepper.shell

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

drivers/stepper/stepper_shell.c

@@ -360,7 +360,7 @@ static int cmd_stepper_enable_constant_velocity_mode(const struct shell *sh, siz
 {
 	const struct device *dev;
 	int err = -EINVAL;
-	enum stepper_direction direction;
+	enum stepper_direction direction = STEPPER_DIRECTION_POSITIVE;
 
 	for (int i = 0; i < ARRAY_SIZE(stepper_direction_map); i++) {
 		if (strcmp(argv[ARG_IDX_PARAM], stepper_direction_map[i].name) == 0) {