iwlwifi: move iwl_get_hw_mode to iwl-core.h

This patch moves iwl_get_hw_mode to iwl-core.h.

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>