iwl3945: workaround for firmware frame tx rejection

Firmware can reject to transmit frame on passive channel, when it
did not yet received any frame with valid CRC on that channel.
Workaround this problem in the driver.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>