USB: EHCI: tegra: set txfill_tuning

To avoid memory fetch underflows with larger USB transfers, Tegra SoCs
need txfill_tuning's txfifothresh register field set to a non-default
value. Add a custom reset override in order to set this up.

These values are recommended practice for all Tegra chips. However,
I've only noticed practical problems when not setting them this way on
systems using Tegra124. Hence, CC: stable only for recent kernels which
actually support Tegra124.

Cc: <stable@vger.kernel.org> # 3.14+
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>