iio: dac: mcp4725: fix odd_ptr_err.cocci warnings

PTR_ERR should access the value just tested by IS_ERR

Generated by: scripts/coccinelle/tests/odd_ptr_err.cocci

CC: Tomas Novotny <tomas@novotny.cz>
Signed-off-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>