mmc: tmio: move TMIO_MASK_{READOP, WRITEOP} handling to correct place

As far as I tested the IP on UniPhier SoCs, TMIO_STAT_{RXRDY,TXRQ}
are asserted for DMA mode as well as for PIO.  I need to disable the
those IRQs in dma_ops->start hook, otherwise the DMA transfer fails
with the following error message:
  PIO IRQ in DMA mode!

Renesas chips are the same cases since I see their dma_ops->start
hooks explicitly clear TMIO_STAT_{RXRDY,TXRQ} (with nice comment!).

If we do this sanity check in TMIO MMC core, RXRDY/TXRQ handling
should be entirely moved to the core.  tmio_mmc_cmd_irq() will
be a suitable place to disable them.

The probe function sets TMIO_MASK_{READOP,WRITEOP} but this is odd.

    /* Unmask the IRQs we want to know about */
    if (!_host->chan_rx)
            irq_mask |= TMIO_MASK_READOP;
    if (!_host->chan_tx)
            irq_mask |= TMIO_MASK_WRITEOP;

At this point, _host->{chan_rx,chan_tx} are _always_ NULL because
tmio_mmc_request_dma() is called after this code.  Consequently,
TMIO_MASK_{READOP,WRITEOP} are set here whether DMA is used or not.
Remove this pointless code.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>