[POWERPC] Convert therm_pm72.c to use the kthread API

This converts the therm_pm72.c driver to use the kthread API.  I
thought about making it use kthread_stop() instead of the `state'
variable and the `ctrl_complete' completion, but that isn't simple and
will require changing the way that `state' is used.

Signed-off-by: Paul Mackerras <paulus@samba.org>