drm/nouveau/debugfs: Use seq_putc() in nouveau_debugfs_pstate_get()

A single character (line break) should be put into a sequence.
Thus use the corresponding function “seq_putc”.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e6f9b34e-58d9-b1aa-8b12-8d4a55153d1d@web.de