atm: nicstar: fix regression made by previous patch

The commit 8390f814 "atm: nicstar: re-use native mac_pton() helper" did a
usefull thing. However, mac_pton() returns 1 in the case of the successfully
parsed input. This patch fixes a typo.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>