iwlegacy: 4965-rs: remove unnecessary null check for sta and lq_sta

both sta and lq_sta are guaranteed to be not null in the calling
function so we don't need to check them here.

Signed-off-by: Greg Dietsche <Gregory.Dietsche@cuw.edu>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>