usb: host: xhci-tegra: Extract firmware enable helper

Extract a helper that enables message generation from the firmware. This
removes clutter from tegra_xusb_probe() and will also come in useful for
subsequent patches that introduce suspend/resume support.

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

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20191206140653.2085561-6-thierry.reding@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>