Add a missing call to iwx_ctxt_info_free_fw_img() in an error path

Add a missing call to iwx_ctxt_info_free_fw_img() in an error path
of iwx_ctxt_info_init() which should always free on error.

Also, free firmware paging DMA memory in case loading firmware has failed.
If we don't free paging on error we hit KASSERT(dram->paging == NULL)
in iwx_init_fw_sec() once we try to load firmware again.  I have hit
this while debugging firmware load failures during suspend/resume.

ok mpi@