net: stmmac: mdio: convert field prep to use field_prep()

net: stmmac: mdio: convert field prep to use field_prep()

Convert the MDIO field preparation to use field_prep(), which removes
the need to store separate mask and shifts. Also convert the clk_csr
value using __ffs() to do the shift as we need to detect overflows
for this.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/E1vy6AE-0000000Btwv-1LM4@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>