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