ohci: Re-implement TD allocation, matching the specification

The initial motivation for doing this is to remove the `used` flag
in the TD. If we use this flag, we end up being required to read from TDs
that the OHCI controller might be modifying (i.e. the OHCI controller
logically owns the TD). This happens when we try to allocate a
new, empty TD while the OHCI host controller is working on a transfer.

Move the `used` flag to `gtd_extra_data_t`. This data is only used
by the CPU, and the OHCI controller never accesses it.

The existing allocation method for TDs does *not* put an empty TD
onto each ED (i.e it does *not* do what is shown in Figure 5-6
of the OHCI specification). Instead, the NextTD field of the last
TD is set to 0. The TailP field of the ED is also set to 0.
This works in many cases. However, this implementation means that
the CPU may end up trying to write to the NextTD field of an
in-progress transfer while the OHCI host controller logically owns it.

Change the implementation to use an empty TD, as suggested by the
specification, for endpoints other than EP0. This avoids the
above issue. It is not necessary to make the change for EP0
because only at most one TD can ever be pending at a time.

The above change should also remove the need for the stall workaround.

In the future, we want to modify the code to access EDs through
an uncached mapping. Because uncached mappings are slow, we want to
access EDs as little as possible. Currently, when a TD completes,
we access an ED in order to figure out the device address and
endpoint number of the TD which was completed.

Because moving `used` to `gtd_extra_data_t` necessitates expanding it,
we have enough room to also store the device address and endpoint number
of the TD. This patch does so.

With the above two changes, we no longer need to access an ED when
a TD completes. Also remove the `index` field from TDs as it is
no longer necessary.

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