xhci: Stop unnecessary tracking of free trbs in a ring

Trying to keep track of free trbs in a ring by adding and subtracting
deltas each time a enqueue or dequeue is increased or moved has proven to
be buggy and complicated, especially over long periods of time.

Recently a bug in counting free trbs was fixed, now taking into account
cancelled URBs that were turned into no-ops, preventing free_trbs to
slowly wander off causing unnecessary ring expansion. See
commit fe82f16a ("xhci: Fix incorrect tracking of free space on
transfer rings")

Turns out its a lot easier to just calculate the numer of free TRB
based on ring size and the current enqueue and dequeue pointer values.
This is currently only needed for the command ring as multi segment
transfer rings already ensures there is enough room the ring during
the ring expansion check.

We could get rid of the ring->num_trbs_free entry completely, but as
the xhci DbC code also uses it we don't clean that up in this patch.

Reported-by: Miller Hunter <MillerH@hearthnhome.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217242


Tested-by: Miller Hunter <MillerH@hearthnhome.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Message-ID: <20230602144009.1225632-8-mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>