rtl8180: add TX queue mapping and support for rtl8187se

This patch adds tx queue mapping for rtl8187se and a long comment
block about their usages.
It adapts the TX function to use that map and it sets properly
the TX descriptor rtl8187se-only fields

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