ixgbe: fix RSS limit for X550

X550 allows for up to 64 RSS queues, but the driver can have max
of 63 (-1 MSIX vector for link).

On systems with >= 64 CPUs the driver will set the redirection table
for all 64 queues which will result in packets being dropped.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>