staging/iio: Use correct argument for sizeof

found with coccicheck
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: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>