Load the transmit dma buffer at attach time as well. We don't need to

Load the transmit dma buffer at attach time as well. We don't need to
load and unload it all the time since the buffer never changes. In
addition, we were loading it with a hardware spin lock held, which
makes the sleepable lock in busdma (for the bounce pages) trigger a
witness warning, as well as ipend being called with it held by uart,
which made it impossible to unload.

These differences don't matter with the v4 busdma implementation, but
they do with the v6 implementation since the latter likes to bounce
transactions more, and will always do so for Atmel's driver.

It's more efficient as well as being more correct.