ixgbe: add get_bus_info method for X550

Added ixgbe_get_bus_info_X550em to X550 code. ixgbe_get_bus_info_X550em
sets bus.width to ixgbe_bus_width_unknown and bus.speed to
ixgbe_bus_speed_unknown, because IOSF does not report a PCIe bus
width or speed.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>