staging:iio:trigger sysfs userspace trigger rework.

Awaiting comments on using the nested_irq_trick so that may change.
Moves away from platform device to sysfs controlled creation and
removal of these triggers.

Fix double free of name on trigger allocation failure thanks
to Michael Hennerich.

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