mmc: sdhi: initialise mmc_data->flags before use

This corrects a logic error that I introduced in
"mmc: sdhi: Add write16_hook"

Reported-by: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>