rtlwifi: remove superfluous condition

If sta == NULL, the changed line will not be reached.
So no need to check that sta != NULL here.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>