term/ns8250: Fix incorrect usage of access_size

The access_size is part of a union, so doesn't technically exist for
a PIO port (i.e., not MMIO), but we set it anyways.

This doesn't cause a bug today because the other leg of the union
doesn't have anything overlapping with it now, but it's bad, I will
punish myself for writing it that way :-) In the meantime, fix this
and actually name the struct inside the union for clarity of intent
and to avoid such issue in the future.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/serial.h

@@ -88,7 +88,7 @@ struct grub_serial_port
   {
     struct
     {
-      bool mmio;
+      bool use_mmio;
       union
       {
 #if defined(__mips__) || defined (__i386__) || defined (__x86_64__)
@@ -96,10 +96,10 @@ struct grub_serial_port
 #endif
         struct
         {
-          grub_addr_t mmio_base;
-          /* Access size uses ACPI definition. */
+          grub_addr_t base;
+          /* Access size uses ACPI definition */
           grub_uint8_t access_size;
-        };
+        } mmio;
       };
     };
     struct