pcnet32: Remove pointless memory barriers

These two memory barriers in performance-critical paths are not needed
on x86.  Even if some other architecture does buffer PCI I/O space
writes, the existing memory-mapped I/O barriers are unlikely to be what
is needed.

Signed-off-by: John Dykstra <john.dykstra1@gmail.com>
Acked-by: Don Fry <pcnet32@verizon.net>
Signed-off-by: David S. Miller <davem@davemloft.net>