usb: xhci: Set accurate add context flags when updating hub attributes

If a USB 3.0 hub is plugged into the root port of the xHC, the xHCI
driver will issue a 'Configure Endpoint' command to the xHC for it
to update its internal data structure for this hub device. The hub
attributes are in the slot context so we need tell xHC to update the
slot context by setting the add context flags of the input control
context to only cover the slot context.

At present the add context flags is or'ed with the slot context bit,
but it should really be accurately set to the slot context, as the
variable that holds the value of the add context flags comes from
whatever was set in the last command execution, which may contain
additional contexts that 'Configure Endpoint' command should not
touch. Some xHC implementations like x86 don't complain such, but
it was observed on Renesas RCar Gen3 platform that the RCar xHC
complains with a 'TRB error' completion codes as the response.

Reported-by: Marek Vasut <marek.vasut@gmail.com>
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>