ixgbe: add helper function for setting RSS key in preparation of X550

Split off the setting of the RSS key into its own function.  This
will help when we add support for X550 which can have different
RSS keys per pool.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>