serial: 8250: Don't delay after transmitter is ready.

The loop in wait_for_xmitr() is delaying one extra uS after the ready
condition has been met.  Rewrite the loop to only delay if the
transmitter is not ready.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>