net: hdlc_fr: remove redundant braces {}

This patch removes redundant braces {}, to fix the
checkpatch.pl warning:
"braces {} are not necessary for any arm 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>