platform/chrome: cros_kbd_led_backlight: fix build warning

drivers/platform/chrome/cros_kbd_led_backlight.c got a new build warning
when using the randconfig in [1]:
>>> warning: unused variable 'keyboard_led_drvdata_ec_pwm'

The warning happens when CONFIG_CROS_EC is set but CONFIG_OF is not set.
Reproduce:
- mkdir build_dir
- wget [1] -O build_dir/.config
- COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 \
  O=build_dir ARCH=s390 SHELL=/bin/bash drivers/platform/chrome/

Fix the warning by using __maybe_unused.  Also use IS_ENABLED() because
CROS_EC is a tristate.

[1]: https://download.01.org/0day-ci/archive/20220717/202207170538.MR39dw8m-lkp@intel.com/config

Fixes: 40f58143 ("platform/chrome: cros_kbd_led_backlight: support EC PWM backend")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220718105047.2356542-1-tzungbi@kernel.org

drivers/platform/chrome/cros_kbd_led_backlight.c

@@ -119,7 +119,7 @@ static const struct keyboard_led_drvdata keyboard_led_drvdata_acpi = {
 
 #endif /* CONFIG_ACPI */
 
-#ifdef CONFIG_CROS_EC
+#if IS_ENABLED(CONFIG_CROS_EC)
 
 static int
 keyboard_led_set_brightness_ec_pwm(struct led_classdev *cdev,
@@ -180,18 +180,18 @@ static int keyboard_led_init_ec_pwm(struct platform_device *pdev)
 	return 0;
 }
 
-static const struct keyboard_led_drvdata keyboard_led_drvdata_ec_pwm = {
+static const __maybe_unused struct keyboard_led_drvdata keyboard_led_drvdata_ec_pwm = {
 	.init = keyboard_led_init_ec_pwm,
 	.brightness_set_blocking = keyboard_led_set_brightness_ec_pwm,
 	.brightness_get = keyboard_led_get_brightness_ec_pwm,
 	.max_brightness = KEYBOARD_BACKLIGHT_MAX,
 };
 
-#else /* CONFIG_CROS_EC */
+#else /* IS_ENABLED(CONFIG_CROS_EC) */
 
-static const struct keyboard_led_drvdata keyboard_led_drvdata_ec_pwm = {};
+static const __maybe_unused struct keyboard_led_drvdata keyboard_led_drvdata_ec_pwm = {};
 
-#endif /* CONFIG_CROS_EC */
+#endif /* IS_ENABLED(CONFIG_CROS_EC) */
 
 static int keyboard_led_probe(struct platform_device *pdev)
 {