mmc: restrict posted write counts for SD cards in CQ mode

Command Queueing requires Write Cache and Power off Notification support
from the card - but using the write cache forms a contract with the host
whereby the card expects to be told about impending power-down.

The implication is that (for performance) the card can do unsafe things
with pending write data - including reordering what gets committed to
nonvolatile storage at what time.

Exposed SD slots and platforms powered by hotpluggable means (i.e.
Raspberry Pis) can't guarantee that surprise removal won't happen.

To limit the scope for cards to invent new ways to trash filesystems,
limit pending writes to 1 (equivalent to the non-CQ behaviour).

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