Revert "serial: 8250_dma: don't bother DMA with small transfers"

This reverts commit 9119fba0.

This commit prevents from sending "big" file using Bluetooth.
When sending a lot of data quickly through the Bluetooth interface, and
after a variable amount of data sent, transfer fails with error:
    kernel: [  415.247453] Bluetooth: hci0 hardware error 0x00

Found on T100TA.

After reverting this commit, send works fine for any file size.

Signed-off-by: Frederic Danis <frederic.danis@linux.intel.com>
Fixes: 9119fba0 (serial: 8250_dma: don't bother DMA with small transfers)
Cc: stable@vger.kernel.org
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>