Fix an issue where the kernel would stall during netboot, showing the
"sched: RT throttling activated" message. This stall was triggered by
the behavior of the mii_interrupt bit (Bit 7 - DP83TD510E_STS_MII_INT)
in the DP83TD510E's PHY_STS Register (Address = 0x10). The DP83TD510E
datasheet (2020) states that the bit clears on write, however, in
practice, the bit clears on read.

This discrepancy had significant implications on the driver's interrupt
handling. The PHY_STS Register was used by handle_interrupt() to check
for pending interrupts and by read_status() to get the current link
status. The call to read_status() was unintentionally clearing the
mii_interrupt status bit without deasserting the IRQ pin, causing
handle_interrupt() to miss other pending interrupts. This issue was most
apparent during netboot.

The fix refrains from using the PHY_STS Register for interrupt handling.
Instead, we now solely rely on the INTERRUPT_REG_1 Register (Address =
0x12) and INTERRUPT_REG_2 Register (Address = 0x13) for this purpose.
These registers directly influence the IRQ pin state and are latched
high until read.

Note: The INTERRUPT_REG_2 Register (Address = 0x13) exists and can also
be used for interrupt handling, specifically for "Aneg page received
interrupt" and "Polarity change interrupt". However, these features are
currently not supported by this driver.

Fixes: 165cd04f ("net: phy: dp83td510: Add support for the DP83TD510 Ethernet PHY")
Cc: <stable@vger.kernel.org>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20230621043848.3806124-1-o.rempel@pengutronix.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>