orinoco: implement set_wiphy_params

... to set fragmentation and RTS thresholds. Also report RTS retry
settings during wiphy init.

Note that the existing semantics for enabling microwave robustness are
preserved on firmwares that have it.

Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>