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

In the Linux kernel, the following vulnerability has been resolved:

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.