MAINTAINERS: fix reference to net phy ABI file

The file sysfs-bus-mdio got removed in favor of sysfs-class-net-phydev,
with contained a duplicated set of information.

Fixes: a6cd0d2d

 ("Documentation: net-sysfs: Remove duplicate PHY device documentation")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>