leds: fix max77693-led build errors

Fix build errors when LEDS_MAX77693=y and V4L2_FLASH_LED_CLASS=m
by restricting LEDS_MAX77693 to =m if V4L2_FLASH_LED_CLASS=m.

drivers/leds/leds-max77693.c:1062: undefined reference to `v4l2_flash_release'
drivers/leds/leds-max77693.c:1068: undefined reference to `v4l2_flash_release'
drivers/built-in.o: In function `max77693_register_led':
drivers/leds/leds-max77693.c:968: undefined reference to `v4l2_flash_init'
drivers/built-in.o: In function `max77693_led_probe':
drivers/leds/leds-max77693.c:1048: undefined reference to `v4l2_flash_release'

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Bryan Wu <cooloney@gmail.com>