ixgbe: convert low_water into an array

Since fc.high_water is an array, we should treat low_water as an array
also. This allows the algorithm to output different values for different
TCs, and then we can distinguish between them. In addition, this patch
changes one path that didn't honor the return value from ixgbe_setup_fc.

Reported-by: Aaron Salter <aaron.k.salter@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>