On pandora, STS_USB interrupt doesn't arrive on USB host cable disconnect
for some reason while VBUS is driven by twl itself, but STS_HW_CONDITIONS
is updated correctly. It does work fine when PHY is powered down though.
To work around that we have to poll.

This patch also moves twl->linkstat update code to callers so that
changes can be handled in thread safe way (as polling work can trigger
at the same time as real irq now).

TI PSP kernels have similar workarounds, so (many?) more boards are likely
affected.
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>