net: phy: mxl-86110: add basic support for led_brightness_set op

net: phy: mxl-86110: add basic support for led_brightness_set op

Add support for forcing each connected LED to be always on or always off
by implementing the led_brightness_set() op.
This is done by modifying the COM_EXT_LED_GEN_CFG register to enable
force-mode and forcing the LED either on or off.
When calling the led_hw_control_set() force-mode is again disabled for
that LED.
Implement mxl86110_modify_extended_reg() locked helper instead of
manually acquiring and releasing the MDIO bus lock for single
__mxl86110_modify_extended_reg() calls.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/58eeefc8c24e06cd2110d3cefbd4236b1a4f44a2.1755884175.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>