greybus: es2: rename USB vendor-request timeout define

Give the USB vendor-request timeout define a more descriptive name.

Also drop the since-long obsolete comments about allowing "the SVC to do
something" and "SVC messages go down our control pipe".

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>