cxgbetool(8): Require and validate only those inputs that are applicable
to the type of rate limiter being configured. For example, the class
WRR scheduler doesn't need any kbps limits (it just needs the weights
for each class), the channel scheduler doesn't need anything except the
aggregate kbps to limit the channel to, and so on.

MFC after: 3 days
Sponsored by: Chelsio Communications