iwlwifi: cleanup spectrum measurement command support

In iwlagn the support for spectrum measurement command has been
disabled since v2.6.29 without any requests for it. In addition to this
when this command is indeed enabled it has been found to trigger firmware
SYSASSERT on at least 4965 and 5100 hardware (see
http://bugzilla.intellinuxwireless.org/show_bug.cgi?id=1952

 ). Since then
this code has been bitrotting and cannot just be enabled without porting.

Remove support for spectrum measurement command from iwlagn. It can be
added back if there is a future need and the firmware problem it triggers
has been fixed. Support for the spectrim measurement notification remains
as it has been enabled all the time.

In addition to this remove the 3945 spectrum measurement command Kconfig
option and make this command always supported. The code added by this
enabling is minimal and only run when user triggers a spectrum measurement
request via sysfs.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>