usb: xhci: remove the code build warning

Initialize the 'err' variate to remove the build warning,
the warning is shown as below:

drivers/usb/host/xhci-tegra.c: In function 'tegra_xusb_mbox_thread':
drivers/usb/host/xhci-tegra.c:552:6: warning: 'err' may be used uninitialized in this function [-Wuninitialized]
drivers/usb/host/xhci-tegra.c:482:6: note: 'err' was declared here

Fixes: e84fce0f ("usb: xhci: Add NVIDIA Tegra XUSB controller driver")
Signed-off-by: Dongjiu Geng <gengdongjiu@huawei.com>
Acked-by: Thierry Reding <treding@nvidia.com>
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>