USB: musb: fix late external abort on suspend

The musb delayed irq work was never flushed on suspend, something which
since 4.9 can lead to an external abort if the work is scheduled after
the grandparent's clock has been disabled:

PM: Suspending system (mem)
PM: suspend of devices complete after 125.224 msecs
PM: suspend devices took 0.132 seconds
PM: late suspend of devices complete after 7.423 msecs
PM: noirq suspend of devices complete after 7.083 msecs
suspend debug: Waiting for 5 second(s).
Unhandled fault: external abort on non-linefetch (0x1008) at 0xd0262c60
...
[<c054880c>] (musb_default_readb) from [<c0547b5c>] (musb_irq_work+0x48/0x220)
[<c0547b5c>] (musb_irq_work) from [<c014f8a4>] (process_one_work+0x1f4/0x758)
[<c014f8a4>] (process_one_work) from [<c014fe5c>] (worker_thread+0x54/0x514)
[<c014fe5c>] (worker_thread) from [<c015704c>] (kthread+0x128/0x158)
[<c015704c>] (kthread) from [<c0109330>] (ret_from_fork+0x14/0x24)

Commit 2bff3916 ("usb: musb: Fix PM for hub disconnect") started
scheduling musb_irq_work with a delay of up to a second and with
retries thereby making this easy to trigger, for example, by suspending
shortly after a disconnect.

Note that we set a flag to prevent the irq work from rescheduling itself
during suspend and instead process a disconnect immediately. This takes
care of the case where we are disconnected shortly before suspending.

However, when in host mode, a disconnect while suspended will still
go unnoticed and thus prevent the controller from runtime suspending
upon resume as the session bit is always set. This will need to be
addressed separately.

Fixes: 550a7375 ("USB: Add MUSB and TUSB support")
Fixes: 467d5c98 ("usb: musb: Implement session bit based runtime PM for musb-core")
Fixes: 2bff3916 ("usb: musb: Fix PM for hub disconnect")
Cc: stable <stable@vger.kernel.org>     # 4.9
Cc: Felipe Balbi <felipe.balbi@linux.intel.com>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>