tty: xuartps: support 64 byte FIFO size

Changes to use the 64 byte FIFO depth and fix the issue
by clearing the txempty interrupt in isr status for tx
after filling in data in start_tx function

Signed-off-by: Suneel Garapati <suneelg@xilinx.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>