net: ngbe: Drop mdiobus_c45_regad()

With the new C45 MDIO access API, there is no encoding of the register
number anymore and thus the masking isn't necessary anymore. Remove it.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>