fbdev: imsttfb: Fix error path of imsttfb_probe()

[ Upstream commit 518ecb6a209f6ff678aeadf9f2bf870c0982ca85 ]

Release ressources when init_imstt() returns failure.

Signed-off-by: Helge Deller <deller@gmx.de>
Stable-dep-of: aba6ab57a910 ("fbdev: imsttfb: fix a resource leak in probe")
Signed-off-by: Sasha Levin <sashal@kernel.org>