atmel_serial: fix hang in set_termios when crtscts is enabled

After enabling hardware flow control, any subsequent termios call may hang
waiting for the transmitter to drain.  This appears to be caused by a
busy-loop in set_termios() waiting for the transmitter to become empty,
which may take a very long time (or hang indefinitely) if the device at
the other end is blocking us.

A quick look through the tty and serial_core code indicates that any
necessary flushing (which is optional) has already been done at this
point, so there's no need for the driver to flush the transmitter on its
own.

Fix it by removing the busy-loop altogether.

Tested-by: Eirik Aanonsen <eaa@wprmedical.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>