hwmon: (lm90) Define status bits

Add bit defines for the status register. And add a function
lm90_is_tripped() which will read status register and return
tripped or not, then lm90_alert can call it directly, and in the
future the IRQ thread also can use it.

[JD: Adjusted to include all the new MAX6696 status flags.]

Signed-off-by: Wei Ni <wni@nvidia.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>