mwl8787: use channel number as .hw_value

Matches firmware expectations.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
mwl8787: fixup scanning

Device now sets enough correct bits to get scan results
back from the firmware.  Still todo: set rates, caps, vendor
IEs, and implement the receive handler.

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