serial: uartps: Fix interrupt mask issue to handle the RX interrupts properly

This patch Correct the RX interrupt mask value to handle the
RX interrupts properly.

Fixes: c8dbdc84 ("serial: xuartps: Rewrite the interrupt handling logic")
Signed-off-by: Nava kishore Manne <nava.manne@xilinx.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>