rtl818x: Explicitly enable contetion window

Currently the contention window enable/disable HW flag is not
touched by the driver.

This patch explicitly set it to the correct value to make sure
contention window is enabled (AFAIK contention window must be
enabled in most (if not all) cases.

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