iwlwifi: move host command sending functions to core module

1. Host command sending functions moved from iwl4965-base.c to iwl-hcmd.c
in iwlcore module
2. enqueue_hcmd function currently stays in iwl4965-base.c. It is invoked
   through the new 'utils' field in priv's ops.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Assaf Krauss <assaf.krauss@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>