iwn(4): various simple fixes

• Fix mbuf leaks in iwn_raw_xmit() and iwn_xmit_task()

(regression since r288178).

• Check IWN_FLAG_RUNNING flag under lock.
• Remove m->m_pkthdr.rcvif initialization (fixed in r283994).
• Enclose some values in return statements into parentheses.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D4069