iwlwifi: refactor iwl_setup_rxon_timing

All callers of iwl_setup_rxon_timing() also send
the command right away, so rename the function
to iwl_send_rxon_timing() and move the sending
into it. Also, some callers clear the data, this
can be done always and thus moved in as well.
Finally, there's no reason for the function to
acquire the spinlock, but it should be called
with the mutex held, so assert that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>