phy: twl4030-usb: make runtime pm more reliable.

A construct like:

        if (pm_runtime_suspended(twl->dev))
               pm_runtime_get_sync(twl->dev);

is against the spirit of the runtime_pm interface as it
makes the internal refcounting useless.

In this case it is also racy, particularly as 'put_autosuspend'
is used to drop a reference.
When that happens a timer is started and the device is
runtime-suspended after the timeout.
If the above code runs in this window, the device will not be
found to be suspended so no pm_runtime reference is taken.
When the timer expires the device will be suspended, which is
against the intention of the code.

So be more direct is taking and dropping references.
If twl->linkstat is VBUS_VALID or ID_GROUND, then hold a
pm_runtime reference, otherwise don't.
Define "cable_present()" to test for this condition.

Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>