hwmon: (pmbus) fix build error unused-function

hwmon: (pmbus) fix build error unused-function

If CONFIG_PMBUS is y and CONFIG_DEBUG_FS is not set.

make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-, will be failed, like this:
drivers/hwmon/pmbus/pmbus_core.c:593:13: error: ‘pmbus_check_block_register’ defined but not used [-Werror=unused-function]
 static bool pmbus_check_block_register(struct i2c_client *client, int page,
             ^~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[3]: *** [drivers/hwmon/pmbus/pmbus_core.o] Error 1
make[2]: *** [drivers/hwmon/pmbus] Error 2
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [drivers/hwmon] Error 2
make[1]: *** Waiting for unfinished jobs....
make: *** [drivers] Error 2

To fix building warning, use __maybe_unused to attach this func.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: c3ffc3a1ff83("hwmon: (pmbus) add a function to check the presence of a block register")
Signed-off-by: Ren Zhijie <renzhijie2@huawei.com>
Link: https://lore.kernel.org/r/20220609120448.139907-1-renzhijie2@huawei.com
[groeck: Fixed continuation line alignment]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>