serial: sh-sci: Fix illegal use of disable_irq_nosync in error interrupt

If error interrupt occurs during the DMA transfer enabled,
the sci_er_interrupt() and sci_rx_interrupt() in sci_mpxed_interrupt()
calls disable_irq_nosync() twice.
This patch avoids called disable_irq_nosync() in sci_er_interrupt().

Signed-off-by: Kazuya Mizuguchi <kazuya.mizuguchi.ks@renesas.com>