eal: fix return codes on thread naming failure

eal: fix return codes on thread naming failure

The doc says this function returns negative errno
on error, but it currently returns either -1 or
positive errno.

It was incorrectly assumed that pthread_setname_np()
returns negative error numbers. It always returns
positive ones, so this patch negates its return value
before returning.

Fixes: 3901ed99c2f8 ("eal: fix thread naming on FreeBSD")
Cc: stable@dpdk.org
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>
Reviewed-by: Olivier Matz <olivier.matz@6wind.com>