xhci: allow bytewise capability register reads

Some guests need this according to
Alejandro Martinez Ruiz <alex@securiforest.com>

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-xhci.c

@@ -2746,8 +2746,10 @@ static void xhci_doorbell_write(void *ptr, target_phys_addr_t reg,
 
 static const MemoryRegionOps xhci_cap_ops = {
     .read = xhci_cap_read,
-    .valid.min_access_size = 4,
+    .valid.min_access_size = 1,
     .valid.max_access_size = 4,
+    .impl.min_access_size = 4,
+    .impl.max_access_size = 4,
     .endianness = DEVICE_LITTLE_ENDIAN,
 };