Make LEDS_LP55XX_COMMON depend on I2C to fix build errors:

leds-lp55xx-common.c:(.text+0x9d): undefined reference to `i2c_smbus_read_byte_data'
leds-lp55xx-common.c:(.text+0x8fc): undefined reference to `i2c_smbus_write_byte_data'

These errors happened when I2C=m and LEDS_LP55XX_COMMON=y, so
prevent that from being possible.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Dan Murphy <dmurphy@ti.com>
Cc: linux-leds@vger.kernel.org
Cc: Milo Kim <milo.kim@ti.com>
Cc: Mathias Nyman <mathias.nyman@nokia.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>