net80211 drivers: fix rate setup for EAPOL frames, obtain Tx parameters
directly from the node.

• Use ni_txparms directly instead of calculating them manually every time
• Move M_EAPOL flag check upper; otherwise it may be skipped due to

'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.

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D9811