staging:iio: Drop the unused buffer enable() and is_enabled() callbacks

Currently none of the buffer implementations implements the enable() or
is_enable() nor does core code try to call these. So it is safe to remove them.
Acked-by: NJonathan Cameron <jic23@kernel.org>
Signed-off-by: NLars-Peter Clausen <lars@metafoo.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/Documentation/ring.txt

@@ -51,7 +51,3 @@ get_bytes_per_datum, set_bytes_per_datum
 get_length / set_length
   Get/set the number of complete scans that may be held by the buffer.
 
-is_enabled
-  Query if ring buffer is in use
-enable
-  Start the ring buffer.

drivers/staging/iio/buffer.h

@@ -31,8 +31,6 @@ struct iio_buffer;
  * @set_bytes_per_datum:set number of bytes per datum
  * @get_length:		get number of datums in buffer
  * @set_length:		set number of datums in buffer
- * @is_enabled:		query if buffer is currently being used
- * @enable:		enable the buffer
  *
  * The purpose of this structure is to make the buffer element
  * modular as event for a given driver, different usecases may require
@@ -58,9 +56,6 @@ struct iio_buffer_access_funcs {
 	int (*set_bytes_per_datum)(struct iio_buffer *buffer, size_t bpd);
 	int (*get_length)(struct iio_buffer *buffer);
 	int (*set_length)(struct iio_buffer *buffer, int length);
-
-	int (*is_enabled)(struct iio_buffer *buffer);
-	int (*enable)(struct iio_buffer *buffer);
 };
 
 /**