firmware: remove warning at documentation generation time

This patch removes following error at for `make htmldocs`. No functional
change.

	./drivers/base/firmware_class.c:1348: WARNING: Bullet list ends without a blank line; unexpected unindent.
Signed-off-by: NSilvio Fricke <silvio.fricke@gmail.com>
Reviewed-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: NLuis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>