i2c: mux: use proper dev when removing "channel-X" symlinks

Those symlinks are created for the mux_dev, so we need to remove it from
there. Currently, it breaks for muxes where the mux_dev is not the device
of the parent adapter like this:

[   78.234644] WARNING: CPU: 0 PID: 365 at fs/sysfs/dir.c:31 sysfs_warn_dup+0x5c/0x78()
[   78.242438] sysfs: cannot create duplicate filename '/devices/platform/i2cbus@8/channel-0'

Remove confusing comments while we are here.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Fixes: c9449aff
Cc: stable@kernel.org