regmap: clairify meaning of max_register

The exact meaning of max_register is not entierly clear. Follow
the common wording and use "address" instead of "index".
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Mark Brown <broonie@kernel.org>

include/linux/regmap.h

@@ -162,7 +162,7 @@ typedef void (*regmap_unlock)(void *);
  *		  This field is a duplicate of a similar file in
  *		  'struct regmap_bus' and serves exact same purpose.
  *		   Use it only for "no-bus" cases.
- * @max_register: Optional, specifies the maximum valid register index.
+ * @max_register: Optional, specifies the maximum valid register address.
  * @wr_table:     Optional, points to a struct regmap_access_table specifying
  *                valid ranges for write access.
  * @rd_table:     As above, for read access.