net: hdlc_ppp: fix the code style issue about "foo* bar"

Fix the checkpatch error as "foo* bar" or "foo*bar" should be "foo *bar".

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>