i2c: muxes: fix wrong use of sizeof(ptr)

sizeof when applied to a pointer typed expression gives the size of
the pointer

The semantic patch that makes this output is available
in scripts/coccinelle/misc/noderef.cocci.

More information about semantic patching is available at
http://coccinelle.lip6.fr/

Signed-off-by: Laurent Navet <laurent.navet@gmail.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>