• Use ni_txparms directly instead of calculating them manually every time;
• Move M_EAPOL flag check upper; otherwise it may be overridden by 'ucastrate' / 'mcastrate' check;
• Use 'mgtrate' for control frames too (see ifconfig(8), mgtrate parameter);
• Add few more M_EAPOL checks where it was missing (zyd(4), ural(4), urtw(4)).
• Few unrelated cleanups.

Tested with:

• Intel 6205 (iwn(4)), STA mode;
• WUSB54GC (rum(4)), HOSTAP mode + RTL8188EU (rtwn(4)), STA mode.