mwl8787: support use_cts_prot bss flag

Needed for HW_HAS_RATE_CONTROL.  Also rename the filter_flags
enum to more accurately describe what it is.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
mwl8787: set the RTS threshold

Signed-off-by: Bob Copeland <me@bobcopeland.com>
mwl8787: set frag threshold

We will need to verify that sequence number is generated correctly
if hardware fragments the frame.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
mwl8787: notify the stack that hw does rate control

Signed-off-by: Bob Copeland <me@bobcopeland.com>