mmc: sd: halt CQHCI before issuing a cache flush command

SD cards perform cache flushes by a CMD49 extension register write -
which needs to be started from the SDHCI command/argument registers and
not a CQHCI slot.

Host access to SD/CQ registers should be exclusive to one or the other,
so issue a halt before doing the command.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>