drop tx_fifo_size

drop tx_fifo_size

... in favor of having what so far was called tx_empty() return the
amount of space available.

Note that in the pl011.c case, original code and comment disagreed, and
I picked the conservative value for it's ->tx_ready() handler's return
value.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>