urtwn: unbreak CTS-to-self protection mode.

Do not set HWRTSEN bit when CTS-to-self is used; CTS2SELF bit triggers
CTS frame transmission by itself (and it does not work when HWRTSEN bit
is set).

Tested with:

• RTL8188CUS, HOSTAP mode (11g)
• RTL8188EU, STA mode (11g)