rtl8180: add rtl8187se fields to TX descriptor

This patch modifies the TX descriptor struct so it can work also
for rtl8187se.
Some reserved field is now meaningful, and where needed union is
used.

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