iwlwifi: remove iwl-wifi.h

This file was recently introduced, but then
directly abused -- it contained private data
that shouldn't have been used by anything
but the implementation of firmware requests
and some very core code. Now that it is no
longer accessed by any code but the code in
iwl-drv.c, we can dissolve it.

Also rename the iwl_nic struct to iwl_drv to
better reflect where and how it is used.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>