usb: musb: dsps: kill OTG timer on suspend

if we don't make sure to kill the timer, it could
expire after we have already gated our clocks.

That will trigger a Data Abort exception because
we would try to access register while clock is gated.

Fix that bug.

Cc: <stable@vger.kernel.org> # v3.14+
Fixes 869c5978 (usb: musb: dsps: add support for suspend and resume)
Tested-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>