iio: triggered-buffer: add {devm_}iio_triggered_buffer_setup_ext variants

iio: triggered-buffer: add {devm_}iio_triggered_buffer_setup_ext variants

This change adds a parameter to the {devm_}iio_triggered_buffer_setup()
functions to assign the extra sysfs buffer attributes that are typically
assigned via iio_buffer_set_attrs().

The functions also get renamed to iio_triggered_buffer_setup_ext() &
devm_iio_triggered_buffer_setup_ext().
For backwards compatibility the old {devm_}iio_triggered_buffer_setup()
functions are now macros wrap the new (renamed) functions with NULL for the
buffer attrs.

The aim is to remove iio_buffer_set_attrs(), so in the
iio_triggered_buffer_setup_ext() function the attributes are assigned
directly to 'buffer->attrs'.

When adding multiple IIO buffers per IIO device, it can be pretty
cumbersome to first allocate a set of buffers, then to dig them out of IIO
to assign extra attributes (with iio_buffer_set_attrs()).

Naturally, the best way would be to provide them at allocation time, which
is what this change does.

At this moment, buffers allocated with {devm_}iio_triggered_buffer_setup()
are the only ones in mainline IIO to call iio_buffer_set_attrs().

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200929125949.69934-4-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>