mac80211: validate user rate mask before configuring driver

Ben reported that when the user rate mask is rejected for not
matching any basic rate, the driver had already been configured.
This is clearly an oversight in my original change, fix this by
doing the validation before calling the driver.

Reported-by: Ben Greear <greearb@candelatech.com>
Fixes: e8e4f528

 ("mac80211: reject/clear user rate mask if not usable")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>