Fix build errors when LEDS_CLASS=m and NET_DSA_HIRSCHMANN_HELLCREEK=y.
This limits the latter to =m when LEDS_CLASS=m.

microblaze-linux-ld: drivers/net/dsa/hirschmann/hellcreek_ptp.o: in function `hellcreek_ptp_setup':
(.text+0xf80): undefined reference to `led_classdev_register_ext'
microblaze-linux-ld: (.text+0xf94): undefined reference to `led_classdev_register_ext'
microblaze-linux-ld: drivers/net/dsa/hirschmann/hellcreek_ptp.o: in function `hellcreek_ptp_free':
(.text+0x1018): undefined reference to `led_classdev_unregister'
microblaze-linux-ld: (.text+0x1024): undefined reference to `led_classdev_unregister'
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Link: lore.kernel.org/r/202101060655.iUvMJqS2-lkp@intel.com
Cc: Kurt Kanzenbach <kurt@linutronix.de>
Link: https://lore.kernel.org/r/20210106021815.31796-1-rdunlap@infradead.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>