drm/mgag200: Return errno codes from PLL compute functions

Return -EINVAL if there's no PLL configuration for the given pixel
clock. The returned errors are currently ignored by the caller, but
the errno codes will become useful when the compute functions run
during atomic checks.

v2:
	* give a rational for this change (Sam)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714142240.21979-3-tzimmermann@suse.de

drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -133,7 +133,7 @@ static int mgag200_g200_set_plls(struct mga_device *mdev, long clock)
 
 	if (clock > p_clk_max) {
 		drm_err(dev, "Pixel Clock %ld too high\n", clock);
-		return 1;
+		return -EINVAL;
 	}
 
 	if (clock < p_clk_min >> 3)
@@ -288,7 +288,7 @@ static int mga_g200se_set_plls(struct mga_device *mdev, long clock)
 
 	if (delta > permitteddelta) {
 		pr_warn("PLL delta too large\n");
-		return 1;
+		return -EINVAL;
 	}
 
 	WREG_MISC_MASKED(MGAREG_MISC_CLKSEL_MGA, MGAREG_MISC_CLKSEL_MASK);