usb: musb: Fix unbalanced pm_runtime_enable

When reloading omap2430 kernel module we get a warning about
unbalanced pm_runtime_enable. Let's fix this. Note that we
need to do this after the child musb-core platform_device is
removed because of pm_runtime_irq_safe being set at the child.

Cc: Bin Liu <b-liu@ti.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: NeilBrown <neil@brown.name>
Reviewed-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>