usb: xhci: tegra: fix runtime PM error handling

The address-of operator will always evaluate to true. However,
power should be explicitly disabled if no power domain is used.

Remove the address-of operator.

Fixes: 58c38116 ("usb: xhci: tegra: Add support for managing powergates")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>