net: hdlc: fix an code style issue about EXPORT_SYMBOL(foo)

According to the chackpatch.pl,
EXPORT_SYMBOL(foo); should immediately follow its function/variable.

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>