staging:iio:meter:ade7758: Use iio channel spec and miscellaneous other changes.

Use iio channel spec for the ring buffer channels.
Add/update file comment/license headers.
Use available_scan_masks to prevent that multiple channels are enabled.
Remove wavefrom type attributes. (no handled directly by the scan_elements)
Use SPI_MODE_1.
Move ade7758_initial_setup() before ade7758_probe_trigger() to ensure the
ADE7758 interrupt is disabled when the host interrupt get's enabled.
Add spi_device_id.
Update ring buffer setup.

Some backporting needed to ensure driver builds all the way through core
changes.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>