ixgbe: fix build with clang

ixgbe: fix build with clang

Clang fails with an error about a variable being used uninitialized:

lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c:67:30:
error: variable 'dma_addr0' is uninitialized
      when used here [-Werror,-Wuninitialized]
                        dma_addr0 = _mm_xor_si128(dma_addr0, dma_addr0);
                                                  ^~~~~~~~~

This error can be fixed by replacing the call to xor which
takes two parameters, by a call to setzero, which does not take any.

Reported-by: Keith Wiles <keith.wiles@windriver.com>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>