iio: inkern: allocate zeroed memory

 Use kzalloc() rather than kmalloc() for initializing the iio_channel structure.
 This patch enables the iio_dev and iio_chan_spec are set to NULL.
 This may prevent the page fault problem because the pointer of iio_chan_spec
 is initialized as NULL.

 The iio_chan_spec is updated only in case that the IIO map has
 specific channel label.
 When the map has no ADC channel label, then the value of iio_chan_spec
 remains as invalid pointer.
 To prevent this problem, the pointer should be initialized as NULL.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>