[iwm] Use mbuf for large firmware commands, like OpenBSD does.

We also need to consider the size of large firmware commands in iwm_alloc_tx_ring(),
in the dma tag creation, when qid == IWM_MVM_CMD_QUEUE. The old code apparently
only allocated a 2KB (MCLBYTES) sized buffer when it actually expected 4KB.

Submitted by: Imre Vadasz <imre@vdsz.com>
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D6824