s3fb: fix error return code in s3_pci_probe()

Fix to return -EINVAL when virtual vertical size smaller than real
instead of 0, as done elsewhere in this function. Also remove dup code.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>