nvme-fabrics: Use seq_putc() in __nvmf_concat_opt_tokens()

Single characters should be put into a sequence.
Thus use the corresponding function “seq_putc”.

This issue was transformed by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Keith Busch <kbusch@kernel.org>