usb: host: xhci-tegra: Avoid a fixed duration sleep

Do not use a fixed duration sleep to wait for the DMA controller to
become ready. Instead, poll the L2IMEMOP_RESULT register for the VLD
flag to determine when the XUSB controller's DMA master is ready.

Based on work by JC Kuo <jckuo@nvidia.com>.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20191206140653.2085561-4-thierry.reding@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>