device create: mtd: convert device_create to device_create_drvdata

device_create() is race-prone, so use the race-free
device_create_drvdata() instead as device_create() is going away.

Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>