net: phy: micrel: Introduce lanphy_modify_page_reg

net: phy: micrel: Introduce lanphy_modify_page_reg

As the name suggests this function modifies the register in an
extended page. It has the same parameters as phy_modify_mmd.
This function was introduce because there are many places in the
code where the registers was read then the value was modified and
written back. So replace all this code with this function to make
it clear.

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Link: https://patch.msgid.link/20250818075121.1298170-3-horatiu.vultur@microchip.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>