net: llc: Correct function name llc_pdu_set_pf_bit() in header

Fix the following make W=1 kernel build warning:

 net/llc/llc_pdu.c:36: warning: expecting prototype for pdu_set_pf_bit(). Prototype was for llc_pdu_set_pf_bit() instead

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>