power: bq27xxx_battery: mark some symbols __maybe_unused

Without the I2C driver, we get a few warnings:

drivers/power/bq27xxx_battery.c:288:12: error: 'bq27xxx_regs' defined but not used [-Werror=unused-variable]
 static u8 *bq27xxx_regs[] = {
            ^
drivers/power/bq27xxx_battery.c:994:12: error: 'bq27xxx_powersupply_init' defined but not used [-Werror=unused-function]
 static int bq27xxx_powersupply_init(struct bq27xxx_device_info *di,
            ^
drivers/power/bq27xxx_battery.c:1029:13: error: 'bq27xxx_powersupply_unregister' defined but not used [-Werror=unused-function]
 static void bq27xxx_powersupply_unregister(struct bq27xxx_device_info *di)
             ^

In mainline kernels, this was addressed by a larger rework in 703df6c0 ("power:
bq27xxx_battery: Reorganize I2C into a module"). We probably don't want this backported
into stable kernels, so instead let's shut up the warnings by marking the symbols
as __maybe_unused.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/power/bq27xxx_battery.c

@@ -285,7 +285,7 @@ static u8 bq27421_regs[] = {
 	0x18,	/* AP		*/
 };
 
-static u8 *bq27xxx_regs[] = {
+static u8 *bq27xxx_regs[] __maybe_unused = {
 	[BQ27000] = bq27000_regs,
 	[BQ27010] = bq27010_regs,
 	[BQ27500] = bq27500_regs,
@@ -991,7 +991,7 @@ static void bq27xxx_external_power_changed(struct power_supply *psy)
 	schedule_delayed_work(&di->work, 0);
 }
 
-static int bq27xxx_powersupply_init(struct bq27xxx_device_info *di,
+static int __maybe_unused bq27xxx_powersupply_init(struct bq27xxx_device_info *di,
 				    const char *name)
 {
 	int ret;
@@ -1026,7 +1026,7 @@ static int bq27xxx_powersupply_init(struct bq27xxx_device_info *di,
 	return 0;
 }
 
-static void bq27xxx_powersupply_unregister(struct bq27xxx_device_info *di)
+static void __maybe_unused bq27xxx_powersupply_unregister(struct bq27xxx_device_info *di)
 {
 	/*
 	 * power_supply_unregister call bq27xxx_battery_get_property which