net: stmmac: mdio: merge stmmac_mdio_read() and stmmac_mdio_write()

net: stmmac: mdio: merge stmmac_mdio_read() and stmmac_mdio_write()

stmmac_mdio_read() and stmmac_mdio_write() are virtually identical
except for the final read in the stmmac_mdio_read(). Handle this as
a flag.

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/E1uu8oC-00000001vos-2JnA@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>