x86/cpu: Simplify extended APIC ID detection on AMD

Both if-branches are under if (boot_cpu_has(X86_FEATURE_APIC)), unify
them.

Also, simplify the test for bits:

- 17 ("ApicExtBrdCst: APIC extended broadcast enable") and
- 18 ("ApicExtId: APIC extended ID enable.")

in "D18F0x68 Link Transaction Control."

No functionality change.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1459837795-2588-3-git-send-email-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/amd.c

@@ -565,14 +565,17 @@ static void early_init_amd(struct cpuinfo_x86 *c)
 	 * can safely set X86_FEATURE_EXTD_APICID unconditionally for families
 	 * after 16h.
 	 */
-	if (boot_cpu_has(X86_FEATURE_APIC) && c->x86 > 0x16) {
-		set_cpu_cap(c, X86_FEATURE_EXTD_APICID);
-	} else if (boot_cpu_has(X86_FEATURE_APIC) && c->x86 >= 0xf) {
-		/* check CPU config space for extended APIC ID */
-		unsigned int val;
-		val = read_pci_config(0, 24, 0, 0x68);
-		if ((val & ((1 << 17) | (1 << 18))) == ((1 << 17) | (1 << 18)))
+	if (boot_cpu_has(X86_FEATURE_APIC)) {
+		if (c->x86 > 0x16)
 			set_cpu_cap(c, X86_FEATURE_EXTD_APICID);
+		else if (c->x86 >= 0xf) {
+			/* check CPU config space for extended APIC ID */
+			unsigned int val;
+
+			val = read_pci_config(0, 24, 0, 0x68);
+			if ((val >> 17 & 0x3) == 0x3)
+				set_cpu_cap(c, X86_FEATURE_EXTD_APICID);
+		}
 	}
 #endif