bfin uart: it will hang when read current y count if not disable dma irq

Add irq disable and enable in bfin_serial_rx_dma_timeout in case of
system hang.

This reverts part of commit 9642dbe7

 ("serial: bfin-uart: avoid
dead lock in rx irq handler in smp kernel").

Signed-off-by: Hao Liang <hliang1025@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>