iwlegacy: off by one in iwl3945_hw_build_tx_cmd_rate()

We use "rate_index" like this:
	rate = iwl3945_rates[rate_index].plcp;
The iwl3945_rates[] array has IWL_RATE_COUNT_3945 elements so the
limit here is off by one.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>