rtlwifi: rtl8821ae: Make sure loop counter is signed on all architectures

The for-loop condition does not work correctly on architectures where
"char" is unsigned. Fix it by using an "int", which may also result in
more efficient code.

Signed-off-by: David Müller <d.mueller@elsoft.ch>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>