wil6210: remove set but not used variable 'start'

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

drivers/net/wireless/ath/wil6210/pm.c: In function 'wil_suspend_keep_radio_on':
drivers/net/wireless/ath/wil6210/pm.c:193:16: warning:
 variable 'start' set but not used [-Wunused-but-set-variable]

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