rtl8180: support for rtl8187se RX descriptors

Currently RX status descriptor and RX command descriptor are
represented using the same struct type.
This patch splits this by introducing different types for
rx status and command descriptor.
Doing this make it possible to handle rtl8187se RX descriptors
easier.
This patch do also this by adding specific cases where needed.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>