net/ixgbe/base: remove unneeded flow control code

net/ixgbe/base: remove unneeded flow control code

Support for flow control auto negotiation is defeatured from X550em_a
fiber when connected to speeds lower than 10Gbps. This patch cleans up
the code path which is mostly dead to begin with.

- add device IDs check in ixgbe_device_supports_autoneg_fc().
- ixgbe_fc_autoneg_fiber_x550em_a() is now used only to pass the user
  settings to ixgbe_fc_enable_generic().
- make sure that start_hw does not fail when setup_fc is not implemented.
- small fix in the debug message when ixgbe_device_supports_autoneg_fc()
  returns false.

Signed-off-by: Xiao Wang <xiao.w.wang@intel.com>
Acked-by: Wenzhuo Lu <wenzhuo.lu@intel.com>