r6040: Restore MDIO clock frequency after MAC reset

commit e3f0cc1a upstream.

A number of users have reported that they were not able to get the PHY
to successfully link up, especially after commit c36757eb ("net:
phy: consider AN_RESTART status when reading link status") where we
stopped reading just BMSR, but we also read BMCR to determine the link
status.

Andrius at NetBSD did a wonderful job at debugging the problem
and found out that the MDIO bus clock frequency would be incorrectly set
back to its default value which would prevent the MDIO bus controller
from reading PHY registers properly. Back when we only read BMSR, if we
read all 1s, we could falsely indicate a link status, though in general
there is a cable plugged in, so this went unnoticed. After a second read
of BMCR was added, a wrong read will lead to the inability to determine
a link UP condition which is when it started to be visibly broken, even
if it was long before that.

The fix consists in restoring the value of the MD_CSR register that was
set prior to the MAC reset.

Link: http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=53494
Fixes: 90f750a8

 ("r6040: consolidate MAC reset to its own function")
Reported-by: Andrius V <vezhlys@gmail.com>
Reported-by: Darek Strugacz <darek.strugacz@op.pl>
Tested-by: Darek Strugacz <darek.strugacz@op.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>