usbnet: smsc95xx: Don't clear read-only PHY interrupt

usbnet: smsc95xx: Don't clear read-only PHY interrupt

Upon receiving data from the Interrupt Endpoint, the SMSC LAN95xx driver
attempts to clear the signaled interrupts by writing "all ones" to the
Interrupt Status Register.

However the driver only ever enables a single type of interrupt, namely
the PHY Interrupt.  And according to page 119 of the LAN950x datasheet,
its bit in the Interrupt Status Register is read-only.  There's no other
way to clear it than in a separate PHY register:

https://www.microchip.com/content/dam/mchp/documents/UNG/ProductDocuments/DataSheets/LAN950x-Data-Sheet-DS00001875D.pdf

Consequently, writing "all ones" to the Interrupt Status Register is
pointless and can be dropped.

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>