usb: host: max3421-hcd: Fix error pointer dereference in probe cleanup

usb: host: max3421-hcd: Fix error pointer dereference in probe cleanup

The kthread_run() function returns error pointers so the
max3421_hcd->spi_thread pointer can be either error pointers or NULL.
Check for both before dereferencing it.

Fixes: 05dfa5c9bc37 ("usb: host: max3421-hcd: fix "spi_rd8" uses dynamic stack allocation warning")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/aJTMVAPtRe5H6jug@stanley.mountain
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>