Input: serio_raw - fix uninitialized variable bug

Recent conversion of the driver to use guard notation for locks and
other resources introduced a bug: "written" variable that is supposed to
hold number of bytes successfully written was not initialized.

Fix it by initializing the variable with 0.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202410201730.ItNhUTIv-lkp@intel.com/


Fixes: 5b53a9d4 ("Input: serio_raw - use guard notation for locks and other resources")
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>