usb: host: xhci-tegra: Fix Wunused-const-variable

Clang produces the following warning

drivers/usb/host/xhci-tegra.c:357:27: warning: unused variable
'mbox_cmd_name' [-Wunused-const-variable]
static const char * const mbox_cmd_name[] = {

Looks like it was intended for logging or debugging, but was
never implemented. Removing mbox_cmd_name.

Cc: clang-built-linux@googlegroups.com
Link: https://github.com/ClangBuiltLinux/linux/issues/533


Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>