net: phy: leds: Deduplicate link LED trigger registration

Refactor phy_led_trigger_register() and deduplicate its functionality
when registering LED trigger for link.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20201027182146.21355-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>