tty: bfin-sport-uart: Rx interrupt is not called always with irq disabled.

Replace local_irq_disable by local_irq_save.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>