x86/bootflag: Micro-optimize sbf_write()

Change parity bit with XOR when !parity instead of masking bit out
and conditionally setting it when !parity.

Saves a couple of bytes in the object file.

Co-developed-by: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250226153709.6370-1-ubizjak@gmail.com

arch/x86/kernel/bootflag.c

@@ -38,9 +38,8 @@ static void __init sbf_write(u8 v)
 	unsigned long flags;
 
 	if (sbf_port != -1) {
-		v &= ~SBF_PARITY;
 		if (!parity(v))
-			v |= SBF_PARITY;
+			v ^= SBF_PARITY;
 
 		printk(KERN_INFO "Simple Boot Flag at 0x%x set to 0x%x\n",
 			sbf_port, v);