i2c: mux: demux-pinctrl: Convert to use sysfs_emit_at() API

Follow the advice of the Documentation/filesystems/sysfs.rst and show()
should only use sysfs_emit() or sysfs_emit_at() when formatting the
value to be returned to user space.

Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
[wsa: proper subject prefix]
Signed-off-by: Wolfram Sang <wsa@kernel.org>