net: stmmac: mdio: provide address register formatter

net: stmmac: mdio: provide address register formatter

Rather than duplicating the logic for filling the PA (MDIO address),
GR (MDIO register/devad), CR (clock range) and GB (busy) fields of the
address register in four locations, provide a helper to do this.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Mohd Ayaan Anwar <quic_mohdayaa@quicinc.com>
Link: https://patch.msgid.link/E1uu8ns-00000001voU-0S7b@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>