rtlwifi: rtl8192cu: Fix error in pointer arithmetic

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>
Cc: Stable <stable@vger.kernel.org> [2.6.39+]
Signed-off-by: John W. Linville <linville@tuxdriver.com>