ixgbe: improve logic in ixgbe_init_mbx_params_pf

Use if/then instead of an all-inclusive case statement.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>