net/smc: Fix ENODATA tests in smc_nl_get_fback_stats()

These functions return negative ENODATA but the minus sign was left out
in the tests.

Fixes: f0dd7bf5

 ("net/smc: Add netlink support for SMC fallback statistics")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Guvenc Gulce <guvenc@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>