iwlwifi: remove unneeded locks from apm_stop() and stop_master()

Since priv->reg_lock was added to keep multi-access register manipulation
atomic, priv->lock protection is no longer needed.  Remove this from
iwl_apm_stop_master() and iwl_apm_stop().

Add warning of timeout when polling for busmaster disablement confirmation,
and some comments.

NOTE:  This is needed to enable use of apm_ops.stop() within iwl_eeprom_init();
priv->lock does not get initialized until after this flow.  See patch
"remove power-wasting calls to apm_ops.init()"

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>