usb: Add support for txfifo threshold

CONFIG_USB_EHCI_TXFIFO_THRESH enables setting of the txfilltuning
field in the EHCI controller on reset.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Remy Bohmer <linux@bohmer.net>
Signed-off-by: Tom Warren <twarren@nvidia.com>