mmc: core: properly init drv_type

When the latest version of parsing the new eMMC bindings was moved from
core.c to mmc.c, it was overlooked that drv_type could be used
uninitialized. Fix it!

Fixes: 6186d06c

 ("mmc: parse new binding for eMMC fixed driver type")
Reported-by: Colin Ian King <colin.king@canonical.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>