powerpc/85xx: Fix pin muxing for second USB controller

On P1022/P1013 second USB controller is muxed with second
Ethernet controller. The current code to enable second USB
fails to properly clear pinmux bits used by ethernet. As a
result, Linux freezes when this controller is used. This
patch fixes the problem.

Signed-off-by: Felix Radensky <felix@embedded-sol.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>