net: phy: micrel: Replace hardcoded pages with defines

The functions lan_*_page_reg gets as a second parameter the page
where the register is. In all the functions the page was hardcoded.
Replace the hardcoded values with defines to make it more clear
what are those parameters.

Reviewed-by: Russell King (Oracle) <[email protected]>
Signed-off-by: Horatiu Vultur <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: HoratiuVultur