drm/nouveau/nvkm/core/firmware: Fix formatting, provide missing param description

... and demote non-conformant kernel-doc header.

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/nouveau/nvkm/core/firmware.c:71: warning: Function parameter or member 'subdev' not described in 'nvkm_firmware_get'
 drivers/gpu/drm/nouveau/nvkm/core/firmware.c:71: warning: Function parameter or member 'fwname' not described in 'nvkm_firmware_get'
 drivers/gpu/drm/nouveau/nvkm/core/firmware.c:71: warning: Function parameter or member 'ver' not described in 'nvkm_firmware_get'
 drivers/gpu/drm/nouveau/nvkm/core/firmware.c:71: warning: Function parameter or member 'fw' not described in 'nvkm_firmware_get'
 drivers/gpu/drm/nouveau/nvkm/core/firmware.c:106: warning: Function parameter or member 'fw' not described in 'nvkm_firmware_put'

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: nouveau@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-22-lee.jones@linaro.org