iwlwifi: dvm: remove unused variable compiler warning in debugfs.c

iwlwifi: dvm: remove unused variable compiler warning in debugfs.c

When compiling with W=1, we get the following warning in debugfs.c:

/home/luca/iwlwifi/stack-dev/drivers/net/wireless/intel/iwlwifi/dvm/debugfs.c: In function ‘iwl_dbgfs_fw_restart_write’:
/home/luca/iwlwifi/stack-dev/drivers/net/wireless/intel/iwlwifi/dvm/debugfs.c:2313:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
  int ret;
      ^

Fix it by ignoring the return value in this case.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>