[POWERPC] Fix mpc834x USB-MPH configuration.

mpc834x USB-MPH configuration got broken by commit
6f442560021aecf08658e26ed9a37e6928ef0fa1. The selection bits in SICRL
should be cleared rather than set to configure the USB MUXes for the MPH.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>