ibmveth: Use dcbf rather than dcbfl

When building for power4, newer binutils don't recognise the "dcbfl"
extended mnemonic.

dcbfl RA, RB is equivalent to dcbf RA, RB, 1.

Switch to "dcbf" to avoid the build error.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>