mmc: sh_mmcif: protect against a theoretical race

The MMC subsystem does not guarantee that host driver .request() and
.set_ios() callbacks are serialised. Such concurrent calls, however,
do not have to be meaningfully supported, drivers just have to make
sure to avoid any severe problems.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Simon Horman <horms@verge.net.au>
Cc: Magnus Damm <damm@opensource.se>
Signed-off-by: Chris Ball <cjb@laptop.org>