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.org


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