rtlwifi: rtl8192cu: Fix error in pointer arithmetic

commit 9473ca6e920a3b9ca902753ce52833657f9221cc upstream.

An error in calculating the offset in an skb causes the driver to read
essential device info from the wrong locations. The main effect is that
automatic gain calculations are nonsense.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>