cxgbe(4): create a separate helper routine to write the global RSS key.

While here, make sure only the PF driver attempts to program the global
RSS key (with options RSS). The VF driver doesn't have access to those
device registers.

Sponsored by: Chelsio Communications

(cherry picked from commit 3cc6f777befc2e494b39a4c0dcd91aaf99267f40)