nfp: flower: Fix a potential leak in nfp_tunnel_add_shared_mac()

ida_simple_get() returns an id between min (0) and max (NFP_MAX_MAC_INDEX)
inclusive.
So NFP_MAX_MAC_INDEX (0xff) is a valid id.

In order for the error handling path to work correctly, the 'invalid'
value for 'ida_idx' should not be in the 0..NFP_MAX_MAC_INDEX range,
inclusive.

So set it to -1.

Fixes: 20cce886

 ("nfp: flower: enable MAC address sharing for offloadable devs")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20220218131535.100258-1-simon.horman@corigine.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>