pxa168fb: fix release function mismatch in probe failure

The driver uses kfree() to release the resource allocated by
framebuffer_alloc(), which does not match.
Use framebuffer_release() instead to fix it.

Fixes: 638772c7 ("fb: add support of LCD display controller on pxa168/910 (base layer)")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205160613.32075-1-hslester96@gmail.com