USB: Fix FSL USB driver on non Open Firmware systems

Commit 126512e3 added support for FSL's USB
controller on powerpc. In this commit the Open Firmware code was selected
and compiled unconditionally.

This breaks on ARM systems from FSL which use the same driver (.i.e. the i.MX
series), because ARM don't have OF support (yet). This patch fixes the problem
by only selecting the OF code on systems with Open Firmware support.

Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Compile-Tested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>