iwlwifi: Disable -Wformat when building with GCC.

[jhb]

Summary

GCC's -Wformat complains about NULL format strings passed to
iwl_fw_dbg_collect_trig (though the function handles NULL format
strings). Curious that upstream iwlwifi in Linux is built with GCC
and explicitly opts into this warning via the __printf() attribute.

[bz]

Yeah, on the other hand they seem to have trouble adding extra options to just the driver -- from my reading between lines; who knows...