USB: ehci-fsl: mpc834x config symbol is PPC_MPC834x, not MPC834x

The config symbol for mpc834x processors is CONFIG_PPC_MPC834x,
not CONFIG_MPC834x.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>