net: phy: smsc: Disable Energy Detect Power-Down in interrupt mode

net: phy: smsc: Disable Energy Detect Power-Down in interrupt mode

Simon reports that if two LAN9514 USB adapters are directly connected
without an intermediate switch, the link fails to come up and link LEDs
remain dark.  The issue was introduced by commit b98a6ac6c8ad ("usbnet:
smsc95xx: Forward PHY interrupts to PHY driver to avoid polling").

The PHY suffers from a known erratum wherein link detection becomes
unreliable if Energy Detect Power-Down is used.  In poll mode, the
driver works around the erratum by briefly disabling EDPD for 640 msec
to detect a neighbor, then re-enabling it to save power.

In interrupt mode, no interrupt is signaled if EDPD is used by both link
partners, so it must not be enabled at all.

We'll recoup the power savings by enabling SUSPEND1 mode on affected
LAN95xx chips in a forthcoming commit.

Fixes: b98a6ac6c8ad ("usbnet: smsc95xx: Forward PHY interrupts to PHY driver to avoid polling")
Reported-by: Simon Han <z.han@kunbus.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/439a3f3168c2f9d44b5fd9bb8d2b551711316be6.1655714438.git.lukas@wunner.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>