iwlwifi: reduce overhead if tracing disabled

Tracing commands builds an array of trace data
items even when the tracepoint is disabled.
Instead, loop in the tracepoint assignment.

This reduces overhead if tracing is compiled
into the driver but not enabled and slightly
reduces overall driver size as well:

   text	   data	    bss	    dec	    hex	filename
 114514	   6509	     48	 121071	  1d8ef	before/iwlwifi.ko
 114189	   6509	     48	 120746	  1d7aa	after/iwlwifi.ko

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Reviewed-by: Max Stepanov <Max.Stepanov@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>