net: phy: smsc: Cache interrupt mask

[ Upstream commit 7e8b617e ]

Cache the interrupt mask to avoid re-reading it from the PHY upon every
interrupt.

This will simplify a subsequent commit which detects hot-removal in the
interrupt handler and bails out.

Analyzing and debugging PHY transactions also becomes simpler if such
redundant reads are avoided.

Last not least, interrupt overhead and latency is slightly improved.

Tested-by: Oleksij Rempel <o.rempel@pengutronix.de> # LAN9514/9512/9500
Tested-by: Ferry Toth <fntoth@gmail.com> # LAN9514
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 58aac3a2 ("net: phy: smsc: fix link up detection in forced irq mode")
Signed-off-by: Sasha Levin <sashal@kernel.org>