ixgbe: cleanup ixgbe_init_mbx_params_pf namespace issue

The function ixgbe_init_mbx_params_pf isn't used unless CONFIG_PCI_IOV
is defined.  This is causing namespace warnings.  So I wrapped its
definition in CONFIG_PCI_IOV too.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>