regulator: ad5398: Fix incorrect power down bit mask

AD5398_SW_POWER_DOWN was defined with a bit position outside the valid
range of the device's 16-bit register. The bitwise operation with an
unsigned short would always evaluate to 0, making the power down check
ineffective.

Update AD5398_SW_POWER_DOWN to use a valid bit position within the
16-bit range of the register.

Fixes: 19d022d ("regulator: ad5398: change enable bit name to improve readibility")
Signed-off-by: Dheeraj Reddy Jonnalagadda <dheeraj.linuxdev@gmail.com>
Link: https://patch.msgid.link/20250206103153.59114-1-dheeraj.linuxdev@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: Dheeraj Reddy Jonnalagadda

drivers/regulator/ad5398.c

@@ -16,7 +16,7 @@
 #include <linux/regulator/machine.h>
 #include <linux/regulator/of_regulator.h>
 
-#define AD5398_SW_POWER_DOWN	BIT(16)
+#define AD5398_SW_POWER_DOWN	BIT(15)
 
 struct ad5398_chip_info {
 	struct i2c_client *client;