rtlwifi: rtl8192cu: Fix more pointer arithmetic errors

This driver uses a number of macros to get and set various fields in the
RX and TX descriptors. To work correctly, a u8 pointer to the descriptor
must be used; however, in some cases a descriptor structure pointer is used
instead. In addition, a duplicated statement is removed.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>