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