net: stmmac: stm32: use PHY capability for TX clock stop

Whether the MII transmit clock can be stopped is primarily a property
of the PHY (there is a capability bit that should be checked first.)
Whether the MAC is capable of stopping the transmit clock is a separate
issue, but this is already handled by the core DesignWare MAC code.

Add the flag to allow the stmmac core to use the PHY capability.

Cc: Christophe Roullier <christophe.roullier@st.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1tsIU0-005vGL-17@rmk-PC.armlinux.org.uk


Signed-off-by: Paolo Abeni <pabeni@redhat.com>