xhci: clear extra bits from slot context when setting max exit latency

If we need to change the max exit latency with a Evaluate Context
command, we copy the old output slot context and use it as input
context for the command. This also copies the dev_state bits which
are supposed to be zero in the input slot context.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>