More fixes to wpi(4), again not by me! Woo!

• Use IEEE80211_F_DATAPAD;
• (c->ic_flags & IEEE80211_CHAN_PASSIVE) -> IEEE80211_IS_CHAN_PASSIVE(c);
• Convert ackfailcnt to int (there is dereference to *(int *) in ieee80211_ratectl_tx_complete());
• Fix & move cleanup to the end in wpi_rx_done();
• Add missed lock in wpi_update_beacon();
• Try to fix powersave.

PR: kern/197143
Submitted by: Andriy Voskoboinyk <s3erios@gmail.com>