tty:serial:msm:Do not restore Rx interrupts in DMA

tty:serial:msm:Do not restore Rx interrupts in DMA

Avoid data corruption issues that result in CRC errors during file
transfers over serial ports at higher baud rates.

The current msm_serial driver masks the FIFO Rx interrupts in
msm_start_rx_dma() since Rx FIFO interrupts are not required in DMA
mode. However, msm_complete_rx_dma() re-enables the Rx FIFO interrupts
which could cause RXSTALE event to be processed when a TXLEV interrupt
occurs.

The following is the sequence of events that could occur resulting in
data corruption.

msm_start_rx_dma -> msm_complete_rx_dma -->
spin_unlock_irqrestore(&port->lock) -->
msm_uart_irq()(For TXLEV interrupt) -->  msm_handle_rx_dm() (Read from
FIFO resulting in data corruption)

The patch fixes the issue by not restoring the RXLEV and RXSTALE
interrupts in msm_complete_rx_dma(). These interrupts are required only
in FIFO mode.

Signed-off-by: Charanya Venkatraman <charanya@codeaurora.org>
Acked-by: Andy Gross <andy.gross@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>