If usbmisc hasn't probed yet, defer the probe.

It's not enough to check if the platform device for the OF node of the
usbmisc has been registered, but it also needs to have been probed
already before we can call imx_usbmisc_init().

This can happen if the order in which devices are probed change due to
async probing or on-demand probing of dependencies.
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>