Bluetooth: Use kthread API in cmtp

kernel_thread() is a low-level implementation detail and
EXPORT_SYMBOL(kernel_thread) is scheduled for removal.
Use the <linux/kthread.h> API instead.

Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>