net: farsync: replace comparison to NULL with "fst_card_array[i]"

According to the chackpatch.pl, comparison to NULL could
be written "fst_card_array[i]".

Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>