ixgbe: Fix FCRTH value in VM-to-VM loopback mode

ixgbe: Fix FCRTH value in VM-to-VM loopback mode

Orabug: 21918732

The 82599 and X540 datasheets require that FCRTH be "set" for Tx
switching (VM-to-VM loopback) but it did not previously specify what
the value should be set to. It has now been determined that
the correct value is RXPBSIZE - (24*1024).

This setting is also required for later devices.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit bc1fc64fd2d9093496e5b04c6d94d26bfa629c9c)
Signed-off-by: Brian Maly <brian.maly@oracle.com>