wil6210: remove set but not used variable 'wdev'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/ath/wil6210/main.c: In function '_wil6210_disconnect':
drivers/net/wireless/ath/wil6210/main.c:407:23: warning:
 variable 'wdev' set but not used [-Wunused-but-set-variable]

It never used since commit ("e1b43407 wil6210: refactor disconnect flow")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>