net: set static variable an initial value in atl2_probe()

cards_found is a static variable, but when it enters atl2_probe(),
cards_found is set to zero, the value is not consistent with last probe,
so next behavior is not our expect.

Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>