drivers/xgi/common: Fix XGI_SetGroup2

This code looks like it was created from a disassembly of some
other driver. Attempt to fix it, without hardware or documentation.

CID 142909: Operands don't affect result (CONSTANT_EXPRESSION_RESULT)

Change-Id: I9b9cadf2acdba73913aad6bbe0d14ad64a652915
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: https://review.coreboot.org/12774
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Timothy Pearson <tpearson@raptorengineeringinc.com>

src/drivers/xgi/common/vb_setmode.c

@@ -3616,7 +3616,7 @@ static void XGI_SetGroup2(unsigned short ModeNo, unsigned short ModeIdIndex,
 				| (tempax & 0x00FF));
 		temp = (tempax & 0xFF00) >> 8;
 	} else {
-		temp = (tempax & 0x00FF) >> 8;
+		temp = (tempax & 0x00FF);
 	}
 
 	xgifb_reg_set(pVBInfo->Part2Port, 0x44, temp);