leds: LP55XX_COMMON needs to depend on LEDS_CLASS

With these kernel configs:
CONFIG_LEDS_CLASS=m
# CONFIG_LEDS_CLASS_MULTICOLOR is not set
CONFIG_LEDS_LP55XX_COMMON=y
CONFIG_LEDS_LP5521=m
CONFIG_LEDS_LP5562=m

leds-lp55xx-common.c has a build error because it is builtin and
calls an interface that is built as a loadable module (due to
LEDS_CLASS=m). By making LEDS_LP55XX_COMMON depend on LEDS_CLASS,
this config combination cannot happen, thus preventing the build error.

ld: drivers/leds/leds-lp55xx-common.o: in function `lp55xx_register_leds':
leds-lp55xx-common.c:(.text+0xc5f): undefined reference to `devm_led_classdev_register_ext'

Fixes: 33b3a561 ("leds: support new LP8501 device - another LP55xx common")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Dan Murphy <dmurphy@ti.com>
Cc: Milo Kim <milo.kim@ti.com>
Cc: linux-leds@vger.kernel.org
Signed-off-by: Pavel Machek <pavel@ucw.cz>