Staging: rt2870: Removal of kernel_thread() API

Replacing the use of kernel_thread() with kthread_run().   But as
kthread_run() returned a task structure, as compared with
kernel_thread() returning a PID, it was found to be more efficient to
store the task structure pointer as a field data instead of PID
pointer.   On top of modifying the field to store task structure
pointer, the initialization of the field (assigned to
THREAD_PID_INIT_VALUE) was also found unnecessary - as no where it is
found to be used.

Signed-off-by: Peter Teoh <htmldeveloper@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>