iwlwifi: remove dead code from iwl_mac_reset_tsf

iwl_mac_reset_tsf will only ever be called in
IBSS mode, so checking for other modes is not
useful and the code that depends on that will
not be executed.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>