serial: fix kernel-doc warning in comments

Fix Sphinx warning in serial_core.c:

../drivers/tty/serial/serial_core.c:1969: WARNING: Definition list ends without a blank line; unexpected unindent.

Fixes: 73abaf87 ("serial: earlycon: Refactor parse_options into serial core")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/e989641c-224a-1090-e596-e7cc800bed44@infradead.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -1964,8 +1964,10 @@ uart_get_console(struct uart_port *ports, int nr, struct console *co)
  *	   console=<name>,io|mmio|mmio16|mmio32|mmio32be|mmio32native,<addr>,<options>
  *
  *	The optional form
+ *
  *	   earlycon=<name>,0x<addr>,<options>
  *	   console=<name>,0x<addr>,<options>
+ *
  *	is also accepted; the returned @iotype will be UPIO_MEM.
  *
  *	Returns 0 on success or -EINVAL on failure