usb: musb: Fix broken use of static variable for multiple instances

We can't use static variable first for checking when musb is
initialized when we have multiple musb instances like on am335x.

Tested-by: Ladislav Michl <ladis@linux-mips.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>