tty: serdev: core: Remove unused variable 'dummy'

No need to have a dummy variable here.

Fixes the following W=1 kernel build warning(s):

 drivers/tty/serdev/core.c: In function ‘serdev_controller_remove’:
 drivers/tty/serdev/core.c:791:6: warning: variable ‘dummy’ set but not used [-Wunused-but-set-variable]

Cc: Rob Herring <robh@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-2-lee.jones@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>