hwmon: (lm90) Rework alarm/status handling

Many chips supported by this driver clear status registers after it
is read and update it in the next measurement cycle. Normally this falls
under the radar because all registers are only read once per measurement
cycle. However, there is an exception: Status registers are always read
during interrupt and laert handling. This can result in invalid status
reports if userspace reads an alarm attribute immediately afterwards.

Rework alarm/status handling by keeping a shadow register with 'current'
alarms, and by ensuring that the register is either only updated once per
measurement cycle or not cleared.

A second problem is related to alert handling: Alert handling is disabled
for chips with broken alert after an alert was reported, but only
re-enabled if attributes are read by the user. This means that alert
conditions may appear and disappear unnoticed. Remedy the situation
by introducing a worker to periodically read the status register(s) while
alert handling is disabled, and re-enable alerts after the alert condition
clears.

Yet another problem is that sysfs and udev events are currently only
reported to userspace if an alarm is raised, but not if an alarm condition
clears. Use the new worker to detect that situation and also generate
sysfs and udev events in that case.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>