staging: emxx_udc: Remove unused device_desc declaration

Clang warns:

drivers/staging/emxx_udc/emxx_udc.c:1373:37: warning: variable
'device_desc' is not needed and will not be emitted
[-Wunneeded-internal-declaration]
static struct usb_device_descriptor device_desc = {
                                    ^
1 warning generated.

This definition hasn't been attached to anything since the driver was
introduced in commit 33aa8d45a4fe ("staging: emxx_udc: Add Emma Mobile
USB Gadget driver") and neither GCC nor Clang emit any reference to the
variable in the final assembly. The only reason GCC doesn't warn about
this variable being unused is the sizeof function.

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>