regulator: fix fatal kernel-doc error

Fix fatal kernel-doc error in <linux/regulator/driver.h>:

Error(include/linux/regulator/driver.h:52): cannot understand prototype: 'struct regulator_linear_range '
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
[Rewrote first line -- broonie]
Signed-off-by: Mark Brown <broonie@linaro.org>

include/linux/regulator/driver.h

@@ -40,6 +40,8 @@ enum regulator_status {
 };
 
 /**
+ * struct regulator_linear_range - specify linear voltage ranges
+ *
  * Specify a range of voltages for regulator_map_linar_range() and
  * regulator_list_linear_range().
  *