drm/exynos: dsi: fix exynos_dsi_set_pll() wrong return value

The type of this function is unsigned long, and it is expected
to return proper fout value or zero if something is wrong.
So this patch fixes wrong return value for error cases.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Acked-by: Inki Dae <inki.dae@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>