net: stmmac: mdio: return clk_csr value from stmmac_clk_csr_set()

Return the clk_csr value from stmmac_clk_csr_set() rather than
using priv->clk_csr, as this struct member now serves very little
purpose. This allows us to remove priv->clk_csr.

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/E1uu8oW-00000001vpH-46zf@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>