phy: fix xa_alloc_cyclic() error handling

xa_alloc_cyclic() can return 1, which isn't an error. To prevent
situation when the caller of this function will treat it as no error do
a check only for negative here.

Fixes: 38496878 ("net: phy: Introduce ethernet link topology representation")
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>