ixgbe: create conversion functions from link_status to bus/speed

This patch cleans up ixgbe_get_bus_info_generic to call some conversion
functions, which are used also in a follow on patch that needs to convert
between the link_status PCIe config values into ixgbe's internal enum
representations.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>