ixgbe: Reorder link flow control functions in ixgbe_common.c

We can avoid many of the forward declarations found in ixgbe_common.c by
just reordering things so this patch does that to help cleanup the code.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>