hwmon: (pmbus/core) Avoid race condition during probe

hwmon: (pmbus/core) Avoid race condition during probe

pmbus_write_smbalert_mask() is not guarded, which can cause race conditions
with concurrent sysfs attribute accesses.

Similar, PMBus accesses in pmbus_init_debugfs() are not guarded, also
resulting in potential race conditions.

Add guards to both functions to prevent the races.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Fixes: 221819ca4c36e ("hwmon: (pmbus/core) Add interrupt support")
Fixes: 1e069dfd96dfe ("hwmon: (pmbus) Add debugfs for status registers")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>