b43: use temporary rate_index for error checking

The mac80211 rate_index changed to be a u8, so
can't hold the negative error value properly.
Use a temporary variable for error checking.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>