tty: serial: sifive: Struct headers should start with 'struct <name>'

Also supply a missing member description.

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

 drivers/tty/serial/sifive.c:157: warning: cannot understand function prototype: 'struct sifive_serial_port '

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: linux-serial@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
Link: https://lore.kernel.org/r/20201104193549.4026187-26-lee.jones@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>