regulator: core: Fix build error due to const qualifier for ops

Drop const qualifier for ops of struct regulator_desc.
Allow regulator drivers to update ops before registering regulator.

Fix below build error:
  CC [M]  drivers/regulator/mc13892-regulator.o
drivers/regulator/mc13892-regulator.c: In function 'mc13892_regulator_probe':
drivers/regulator/mc13892-regulator.c:586:3: error: assignment of member 'set_mode' in read-only object
drivers/regulator/mc13892-regulator.c:588:3: error: assignment of member 'get_mode' in read-only object
make[2]: *** [drivers/regulator/mc13892-regulator.o] Error 1
make[1]: *** [drivers/regulator] Error 2
make: *** [drivers] Error 2

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

include/linux/regulator/driver.h

@@ -246,7 +246,7 @@ struct regulator_desc {
 	int id;
 	bool continuous_voltage_range;
 	unsigned n_voltages;
-	const struct regulator_ops *ops;
+	struct regulator_ops *ops;
 	int irq;
 	enum regulator_type type;
 	struct module *owner;