net: hdlc_fr: add braces {} to all arms of the statement

Braces {} should be used on all arms of this statement.

Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>