rtlwifi: rtl8192cu: Convert driver to use common macros

These drivers use a set of complicated macros to extract and insert
information for the RX and TX descriptors. Driver rtl8192cu had a
different set than was used for the PCI-based drivers. To simplify
the code, rtl8192cu is switched to use the common version. In the
process, two errors in those common macros were found and fixed.

Besides simplifying the code, there is an additional benefit. We have
no BE hardware to test the PCI driver, but using the common macros
provides an additional test for the validity of many endian-sensitive
operations.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>