regmap: correct the description of structure element in reg_field

Fix incorrect description of structure element "msb", which is
described as "reg".

Signed-off-by: Bintian Wang <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

include/linux/regmap.h

···
  *
  * @reg: Offset of the register within the regmap bank
  * @lsb: lsb of the register field.
- * @reg: msb of the register field.
+ * @msb: msb of the register field.
  * @id_size: port size if it has some ports
  * @id_offset: address offset for each ports
  */