iwlwifi: fix incorrect monitor mode operation

This patch fixes monitor mode operation for iwlwifi.

Problems addressed:
1. when monitor mode was enabled, multiple, overlapped calls were being
made to 'iwl3945_bg_set_monitor'

2. when monitor mode was disabled (via the configure_filter callback),
the driver was still enabling monitor mode

3. when monitor mode was enabled, the selected channel was not set
(eg. 'iwconfig wlanx mode monitor channel n' DID NOT SET channel 'n'
when packet capture was subsequently enabled)

Signed-off-by: Rick Farrington <rickdic@hotmail.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>