greybus: es2: test apb1_log_task safely

When usb_log_enable() is called, the global apb1_log_task is used to
hold the result of kthread_run().  It is possible for kthread_run()
to return an error pointer, so tests of apb_log_task against NULL
are insufficient to determine its validity.

Note that kthread_run() never returns NULL so we don't have to check
for that.  But apb1_log_task is initially NULL, so that global must
be both non-null and not an error in order to be considered valid.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>