igb: cleanup flow control configuration to make requested/current more clear

This patch cleans up the flow control configuration for igb to make it a
bit more readable in regards to what the requested and current modes are.
This should help with the maintainability of the current igb driver in
regards to flow control.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>