fbdev: sm712fb: set error code in probe

If smtcfb_pci_probe() does not detect a valid chip it cleans up
everything and returns 0. This can result in various bad things later.
The patch sets the error code on the corresponding path.

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

Signed-off-by: Evgeny Novikov <novikov@ispras.ru>
Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Teddy Wang <teddy.wang@siliconmotion.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200706155328.8396-1-novikov@ispras.ru