term/ns8250: Fix incorrect usage of access_size

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>