netfilter: nf_log: Replace a seq_printf() call by seq_puts() in seq_show()

A string which did not contain a data format specification should be put
into a sequence. Thus use the corresponding function “seq_puts”.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>