mmc: sh_mmcif: fix I/O errors

The INT_BUFWEN IRQ often arrives with other bits set too. If they are not
cleared, an additional IRQ can be triggered, sometimes also after the MMC
request has already been completed. This leads to block I/O errors. Earlier
Teppei Kamijou also observed these additional interrupts and proposed to
explicitly wait for them. This patch chooses an alternative approach of
clearing all active bits immediately, when processing the main interrupt.

Reported-by: Teppei Kamijou <teppei.kamijou.yb@renesas.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>