net: phy: at803x: implement generic .handle_interrupt() callback

In an attempt to actually support shared IRQs in phylib, we now move the
responsibility of triggering the phylib state machine or just returning
IRQ_NONE, based on the IRQ status register, to the PHY driver. Having
3 different IRQ handling callbacks (.handle_interrupt(),
.did_interrupt() and .ack_interrupt() ) is confusing so let the PHY
driver implement directly an IRQ handler like any other device driver.
Make this driver follow the new convention.

Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Cc: Michael Walle <michael@walle.cc>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Tested-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>