drm/nouveau/volt/gk20a: don't misuse kernel-doc comments

Change kernel-doc "/**" comments to common "/*" comments to prevent
kernel-doc warnings:

gk20a.c:49: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * cvb_mv = ((c2 * speedo / s_scale + c1) * speedo / s_scale + c0)
gk20a.c:49: warning: missing initial short description on line:
 * cvb_mv = ((c2 * speedo / s_scale + c1) * speedo / s_scale + c0)
gk20a.c:62: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * cvb_t_mv =
gk20a.c:62: warning: missing initial short description on line:
 * cvb_t_mv =

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Danilo Krummrich <dakr@redhat.com>
Cc: nouveau@lists.freedesktop.org
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231231233633.6596-4-rdunlap@infradead.org