ixgbe: Update the logic for ixgbe_cache_ring_dcb and DCB RSS configuration

This change cleans up some of the logic in an attempt to try and simplify
things for how we are configuring DCB w/ RSS.

In this patch I basically did 3 things.  I updated the logic for getting
the first register index.  I applied the fact that all TCs get the same
number of queues to simplify the looping logic in caching the DCB ring
register.  Finally I updated how we configure the RQTC register to match
the fact that all TCs are assigned the same number of queues.

Cc: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>