urtwn(4): refactor and fix TX path.

• Split urtwn_tx_start() into urtwn_tx_data() and urtwn_tx_start() (the last will be used for beacon updates / raw xmit path).
• Remove unneeded code from _urtwn_getbuf().
• Use CCK11 for data frames in 11b mode.
• Send EAPOL frames at 1 Mbps.
• Reduce code duplication in urtwn_tx_data().
• Fix sequence numbering.
• Add IEEE80211_RADIOTAP_F_WEP flag for encrypted frames.
• Check URTWN_RUNNING flag under lock.

Tested with RTL8188EU, STA mode.

Reviewed by: kevlo
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D4017