mmc: meson-mx-sdio: Set MMC_CAP_WAIT_WHILE_BUSY

The Meson SDIO controller uses the DAT0 lane for hardware busy
detection. Set MMC_CAP_WAIT_WHILE_BUSY accordingly. This fixes
the following error observed with Linux 5.7 (pre-rc-1):
  mmc1: Card stuck being busy! __mmc_poll_for_busy
  blk_update_request: I/O error, dev mmcblk1, sector 17111080 op
   0x3:(DISCARD) flags 0x0 phys_seg 1 prio class 0

Fixes: ed80a13b

 ("mmc: meson-mx-sdio: Add a driver for the Amlogic Meson8 and Meson8b SoCs")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200416183513.993763-2-martin.blumenstingl@googlemail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>