USB: core: Fix unused variable warning in usb_alloc_dev()

USB: core: Fix unused variable warning in usb_alloc_dev()

The kernel test robot reported that a recent commit caused a "variable
set but not used" warning.  As a result of that commit, the variable
no longer serves any purpose; it should be removed.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308092350.HR4PVHUt-lkp@intel.com/
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Fixes: 1e4c574225cc ("USB: Remove remnants of Wireless USB and UWB")
Link: https://lore.kernel.org/r/7223cc66-f006-42ae-9f30-a6c546bf97a7@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>