tty: serial: st-asc: Demote a kernel-doc formatting abuse

tty: serial: st-asc: Demote a kernel-doc formatting abuse

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

 drivers/tty/serial/st-asc.c:73: warning: expecting prototype for Some of status comes from higher bits of the character and some come from(). Prototype was for ASC_RXBUF_DUMMY_RX() instead

Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-serial@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210520121906.3468725-10-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>