FSP: Reset the response outstanding bitmap on reset start

Once we have the FSP in reset, we don't expect it to respond to any
outstanding mbox commands for which we are expecting one. Reset the
bitmap to reflect the same.

While there, remove the stale comment about timeout resets. We use
the fsp_cmdclass->timeout as a constant for all classes and a value
0 indicates an invalid class entry in the discontiguous fsp_cmdclass[].
The bitmap is what gates whether we process outstanding messages in
the class.

Signed-off-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>