ixgbe: fix ixgbe_setup_mac_link_82599 autoc variables

This patch fixes flow control autonegotiation for KR/KX/K4 interfaces.
When setting up MAC link, the cached autoc value and current autoc value
were being incorrectly used to determine whether link reset is required.
This resulted in the driver ignoring and discarding flow control
negotiation changes that occur since the caching happened, as well as
when the mac was being setup.

This patch also splits the assignments for the 3 autoc variables into
their own block, and adds a comment explaining what each one means, in
order to help keep logic more straightforward while reading the code.

CC: Arun Sharma <asharma@fb.com>
Reported-by: Sourav Chatterjee <sourav.chatterjee@intel.com>
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>