serial: bfin-uart: Don't access tty circular buffer in TX DMA interrupt after it is reset.

When kernel reboot, tty circular buffer is reset before last TX DMA interrupt is called,
while the buffer tail is updated in TX DMA interrupt handler. So, don't update the buffer
tail if it is reset.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>