qca_spi: Improve SPI thread creation

Directly storing the result of kthread_run within the private
driver data makes it harder to identify if the pointer has a
running thread or not. So better use a local variable for
the result check and we don't have to care about error pointer
in the rest of the code.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: David S. Miller <davem@davemloft.net>