fbdev: imsttfb: fix double free in probe()

The init_imstt() function calls framebuffer_release() on error and then
the probe() function calls it again.  It should only be done in probe.

Fixes: 518ecb6a

 ("fbdev: imsttfb: Fix error path of imsttfb_probe()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Helge Deller <deller@gmx.de>