ixgbe: fix infinite recursion on PCIe link down

In some corner cases the functions ixgbe_clear_rar_generic and
ixgbe_clear_vmdq_generic may call one another leading to infinite
recursion.

When ixgbe_clear_vmdq_generic is called with IXGBE_CLEAR_VMDQ_ALL
flag, it's going to clear MPSAR registers, and proceed to call
ixgbe_clear_rar_generic, which in turn will clear the RAR registers,
and recursively call back ixgbe_clear_vmdq_generic. Normally, the
latter would detect that MPSAR registers have already been cleared
and terminate the recursion.

However, when PCIe link is down, and before the driver has had the
opportunity to shut itself down, all register reads return 0xFFFFFFFF,
and all register writes fail silently. In such case, because
ixgbe_clear_vmdq_generic blindly assumes that clearing MPSAR registers
succeeded, it's going to always call ixgbe_clear_rar_generic, which
in turn will always call back ixgbe_clear_vmdq_generic, creating
infinite recursion.

This patch re-reads MPSAR register values after they had been cleared.
In case of PCIe link failure, the values read will be non-zero, which
will terminate the recursion. On the other hand, under normal
circumstances the value read from MPSAR registers is going to be equal
to the value previously written, so this patch is expected not to cause
any regressions.

Signed-off-by: Robert Konklewski <robertx.konklewski@intel.com>
Signed-off-by: Guinan Sun <guinanx.sun@intel.com>
Reviewed-by: Wei Zhao <wei.zhao1@intel.com>

Approved by: imp
Obtained from: DPDK (2d04b9e856125197ec8e967471426d56ab7efcf0)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31621