[xsigo] Avoid storing unused uninitialised fields in gateway address

[xsigo] Avoid storing unused uninitialised fields in gateway address

As reported by Coverity, xsmp_rx_xve_modify() currently passes a
partially initialised struct ib_address_vector to xve_update_tca() and
thence to eoib_set_gateway(), which uses memcpy() to store the whole
structure including the (unused and unneeded) uninitialised fields.

Silence the Coverity warning by zeroing the whole structure.

Signed-off-by: Michael Brown <mcb30@ipxe.org>