iwlwifi: move disable/enable interrupts to iwl-core.c

This patch moves iwl_enable_interrupts and iwl_disable_interrupts
functions to iwl-core.c

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>