iio: dac: mcp4725: Remove unneeded conversions to bool

Found with scripts/coccinelle/misc/boolconv.cocci.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>