hecubafb: add module_put on error path in hecubafb_probe()

In hecubafb_probe(), after a successful try_module_get, vzalloc may
fail and make the hecubafb_probe return, but the module is not put on
this error path.

This patch adds an exit point that calls module_put in such situation.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Pavel Shved <shved@ispras.ru>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>