USB: OHCI: ohci_init_driver(): sanity check overrides

Check for non-NULL overrides before dereferencing since platforms may
pass in NULL overrides.

Signed-off-by: Kevin Hilman <khilman@linaro.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>