iwlwifi: split the drivers for agn and legacy devices 3945/4965

Intel WiFi devices 3945 and 4965 now have their own driver in the folder

	drivers/net/wireless/iwlegacy

Add support to build these drivers independently of the driver for
AGN devices. Selecting the 3945 builds iwl3945.ko and iwl_legacy.ko,
and selecting the 4965 builds iwl4965.ko and iwl_legacy.ko. iwl-legacy.ko
contains code shared between both devices.

The 3945 is an ABG/BG device, with no support for 802.11n. The 4965 is a 2x3
ABGN device.

Signed-off-by: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Acked-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>