wil6210: wmi: Remove useless code

Fix the following whitescan warning:

An unsigned value can never be less than 0.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1617788766-91433-1-git-send-email-jiapeng.chong@linux.alibaba.com