net: phy: add a PHY write barrier when disabling interrupts

net: phy: add a PHY write barrier when disabling interrupts

MDIO bus controllers are not required to wait for write transactions to
complete before returning as synchronization is often achieved by polling
status bits.

This can cause issues when disabling interrupts since an interrupt could
fire before the interrupt handler is unregistered and there's no status
bit to poll.

Add a phy_write_barrier() function and use it in phy_disable_interrupts()
to fix this issue. The write barrier just reads an MII register and
discards the value, which is enough to guarantee that previous writes have
completed.

Signed-off-by: Charles Perry <charles.perry@microchip.com>
Link: https://patch.msgid.link/20260408131821.1145334-4-charles.perry@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>