r8169: fix building with CONFIG_LEDS_CLASS=m

When r8169 is built-in but LED support is a loadable module, the new
code to drive the LED causes a link failure:

ld: drivers/net/ethernet/realtek/r8169_leds.o: in function `rtl8168_init_leds':
r8169_leds.c:(.text+0x36c): undefined reference to `devm_led_classdev_register_ext'

LED support is an optional feature, so fix this issue by adding a Kconfig
symbol R8169_LEDS that is guaranteed to be false if r8169 is built-in
and LED core support is a module. As a positive side effect of this change
r8169_leds.o no longer is built under this configuration.

Fixes: 18764b88

 ("r8169: add support for LED's on RTL8168/RTL8101")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312281159.9TPeXbNd-lkp@intel.com/
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org> # build-tested
Tested-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/d055aeb5-fe5c-4ccf-987f-5af93a17537b@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>