x86: handle both NMI kinds if they occur simultaneously

We shouldn't assume PCI SERR excludes IOCHK.

Once at it, also remove the doubly redundant range restriction on
"reason" - the variable already is "unsigned char".

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -3201,10 +3201,10 @@ void do_nmi(struct cpu_user_regs *regs)
         reason = inb(0x61);
         if ( reason & 0x80 )
             pci_serr_error(regs);
-        else if ( reason & 0x40 )
+        if ( reason & 0x40 )
             io_check_error(regs);
-        else if ( !nmi_watchdog )
-            unknown_nmi_error(regs, (unsigned char)(reason&0xff));
+        if ( !(reason & 0xc0) && !nmi_watchdog )
+            unknown_nmi_error(regs, reason);
     }
 }