iwlwifi: cleanup Kconfig and ifdefs to split 3945 and 4965

Currently the iwl3945 & iwl4965 drivers share some common Kconfig
symbols.  This split it up into options for the individual drivers
and gets rid of all the CONFIG_IWLWIFI cruft.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>