Be slightly more paranoid with the TX DMA buffer maximum threshold.

Specifically - never jack the TX FIFO threshold up to the absolute
maximum; always leave enough space for two DMA transactions to
appear.

This is a paranoia from the Linux ath9k driver.  It can't hurt.

Obtained from:	Linux ath9k


git-svn-id: svn+ssh://svn.freebsd.org/base/head@247092 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f