serial: 8250: replace snprintf in show functions with sysfs_emit

coccicheck complains about the use of snprintf() in sysfs show
functions:
WARNING use scnprintf or sprintf

Use sysfs_emit instead of scnprintf or sprintf makes more sense.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Jing Yao <yao.jing2@zte.com.cn>
Link: https://lore.kernel.org/r/20211104114754.30983-1-yao.jing2@zte.com.cn


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