ath: fix memory disclosure from ath_btcoex_ioctl

The ath_btcoex_ioctl handler allocated a buffer without M_ZERO and
returned it to userland without writing to it.

The device has permissions only for root so this is not urgent, and the
fix can be MFCd and considered for a future EN.

Reported by: Ilja van Sprundel <ivansprundel@ioactive.com>
Submitted by: Domagoj Stolfa <domagoj.stolfa@gmail.com>
Reviewed by: adrian
MFC after: 1 week