mtd: mtd_[read|write|erase]: check for valid input data
mtd_[read|write|erase] are input functions to the mtd subsystem, so
check for valid input data here rather than relying on the drivers doing
this. The checks are copied from the Kernel as of 4.5-rc5
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>