USB: serial: keyspan_pda: remove unused variable

Remove an unused variable which was mistakingly left by commit
37faf5061541 ("USB: serial: keyspan_pda: fix write-wakeup
use-after-free") and only removed by a later change.

This is needed to suppress a W=1 warning about the unused variable in
the stable trees that the build bots triggers.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>