usb: musb: Implement session bit based runtime PM for musb-core

We want to keep musb enabled always when the session bit is
set. This simplifies the PM runtime and allows making it more
generic across the various glue layers.

So far the only exception to just following the session bit is
host mode disconnect where the session bit stays set.

In that case, just allow PM and let the PM runtime autoidle
timeout deal with it.

Signed-off-by: Tony Lindgren <tony@atomide.com>
[b-liu@ti.com: changed using dev_dbg() to musb_dbg()]
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>