serial: 8250_aspeed_vuart: replace snprintf with sysfs_emit

coccinelle report:
./drivers/tty/serial/8250/8250_aspeed_vuart.c:85:8-16:
WARNING: use scnprintf or sprintf
./drivers/tty/serial/8250/8250_aspeed_vuart.c:174:8-16:
WARNING: use scnprintf or sprintf
./drivers/tty/serial/8250/8250_aspeed_vuart.c:127:8-16:
WARNING: use scnprintf or sprintf

Use sysfs_emit instead of scnprintf or sprintf makes more sense.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Yang Guang <yang.guang5@zte.com.cn>
Signed-off-by: David Yang <davidcomponentone@gmail.com>
Link: https://lore.kernel.org/r/fed40753603dac4d14b17970c88e6f5f936348c1.1644541843.git.yang.guang5@zte.com.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>