airo: remove "basic_rate" module option

The "basic_rate" module option is not implemented correctly.  If the
rate was set to zero it was supposed to set it to "basic_rate | 0x80".
Unfortunately the check to see if what zero was wrong and it checked
"!ai->config.rates" (which is always false) instead of
"!ai->config.rates[i]".

This option was just used for development and it wasn't documented
anywhere.  Instead of fixing it, we can just remove it.

Reported-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>