net: phy: mscc: Simplify LED mode update using phy_modify()

net: phy: mscc: Simplify LED mode update using phy_modify()

The vsc85xx_led_cntl_set() function currently performs a manual
read-modify-write sequence protected by the PHY lock to update the
LED mode register (MSCC_PHY_LED_MODE_SEL).

Replace this sequence with a call to phy_modify(), which already
handles read-modify-write operations with proper locking inside
the PHY core.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20251112135715.1017117-2-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>