x86/xen: simplify sysenter and syscall setup

xen_enable_sysenter() and xen_enable_syscall() can be simplified a lot.

While at it, switch to use cpu_feature_enabled() instead of
boot_cpu_has().
Signed-off-by: Juergen Gross <jgross@suse.com>

arch/x86/xen/setup.c

@@ -910,17 +910,9 @@ static int register_callback(unsigned type, const void *func)
 
 void xen_enable_sysenter(void)
 {
-	int ret;
-	unsigned sysenter_feature;
-
-	sysenter_feature = X86_FEATURE_SYSENTER32;
-
-	if (!boot_cpu_has(sysenter_feature))
-		return;
-
-	ret = register_callback(CALLBACKTYPE_sysenter, xen_entry_SYSENTER_compat);
-	if(ret != 0)
-		setup_clear_cpu_cap(sysenter_feature);
+	if (cpu_feature_enabled(X86_FEATURE_SYSENTER32) &&
+	    register_callback(CALLBACKTYPE_sysenter, xen_entry_SYSENTER_compat))
+		setup_clear_cpu_cap(X86_FEATURE_SYSENTER32);
 }
 
 void xen_enable_syscall(void)
@@ -934,12 +926,9 @@ void xen_enable_syscall(void)
 		   mechanism for syscalls. */
 	}
 
-	if (boot_cpu_has(X86_FEATURE_SYSCALL32)) {
-		ret = register_callback(CALLBACKTYPE_syscall32,
-					xen_entry_SYSCALL_compat);
-		if (ret != 0)
-			setup_clear_cpu_cap(X86_FEATURE_SYSCALL32);
-	}
+	if (cpu_feature_enabled(X86_FEATURE_SYSCALL32) &&
+	    register_callback(CALLBACKTYPE_syscall32, xen_entry_SYSCALL_compat))
+		setup_clear_cpu_cap(X86_FEATURE_SYSCALL32);
 }
 
 static void __init xen_pvmmu_arch_setup(void)