x86, setup: fix the setting of 480-line VGA modes

Impact: fix rarely-used feature

The VGA Miscellaneous Output Register is read from address 0x3CC but
written to address 0x3C2.  This was missed when this code was
converted from assembly to C.  While we're at it, clean up the code by
making the overflow bits and the math used to set the bits explicit.
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/boot/video-vga.c

@@ -129,16 +129,20 @@ u16 vga_crtc(void)
 	return (inb(0x3cc) & 1) ? 0x3d4 : 0x3b4;
 }
 
-static void vga_set_480_scanlines(int end)
+static void vga_set_480_scanlines(int lines)
 {
-	u16 crtc;
-	u8  csel;
+	u16 crtc;		/* CRTC base address */
+	u8  csel;		/* CRTC miscellaneous output register */
+	u8  ovfw;		/* CRTC overflow register */
+	int end = lines-1;
 
 	crtc = vga_crtc();
 
+	ovfw = 0x3c | ((end >> (8-1)) & 0x02) | ((end >> (9-6)) & 0x40);
+
 	out_idx(0x0c, crtc, 0x11); /* Vertical sync end, unlock CR0-7 */
 	out_idx(0x0b, crtc, 0x06); /* Vertical total */
-	out_idx(0x3e, crtc, 0x07); /* Vertical overflow */
+	out_idx(ovfw, crtc, 0x07); /* Vertical overflow */
 	out_idx(0xea, crtc, 0x10); /* Vertical sync start */
 	out_idx(end,  crtc, 0x12); /* Vertical display end */
 	out_idx(0xe7, crtc, 0x15); /* Vertical blank start */
@@ -146,24 +150,24 @@ static void vga_set_480_scanlines(int end)
 	csel = inb(0x3cc);
 	csel &= 0x0d;
 	csel |= 0xe2;
-	outb(csel, 0x3cc);
+	outb(csel, 0x3c2);
 }
 
 static void vga_set_80x30(void)
 {
-	vga_set_480_scanlines(0xdf);
+	vga_set_480_scanlines(30*16);
 }
 
 static void vga_set_80x34(void)
 {
 	vga_set_14font();
-	vga_set_480_scanlines(0xdb);
+	vga_set_480_scanlines(34*14);
 }
 
 static void vga_set_80x60(void)
 {
 	vga_set_8font();
-	vga_set_480_scanlines(0xdf);
+	vga_set_480_scanlines(60*8);
 }
 
 static int vga_set_mode(struct mode_info *mode)