net-sysfs: Fix memory leak in XPS configuration

This patch reorders the error cases in showing the XPS configuration so
that we hold off on memory allocation until after we have verified that we
can support XPS on a given ring.

Fixes: 184c449f ("net: Add support for XPS with QoS via traffic classes")
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>