acpi: rephrase comment

"only upto" is not proper English.
Say "up to" and drop "only".
Reported-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

include/hw/acpi/acpi.h

@@ -27,8 +27,7 @@
 #include "hw/irq.h"
 
 /*
- * current device naming scheme supports
- * only upto 256 memory devices
+ * current device naming scheme supports up to 256 memory devices
  */
 #define ACPI_MAX_RAM_SLOTS 256