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 <bintian.wang@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

include/linux/regmap.h

@@ -468,7 +468,7 @@ bool regmap_reg_in_ranges(unsigned int reg,
  *
  * @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
  */