usb: xhci: Initialize dev_state to 0 in the input slot context

Per xHCI spec chapter 6.2.2 table 6-7, as input, software shall
initialize the dev_state field to '0'. Though this does not seem
to cause any issue with most xHC implementations, let's do this
to conform with the spec.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Marek Vasut <marek.vasut@gmail.com>
Tested-by: Matthias Blankertz <matthias.blankertz@cetitec.com>