mtd-utils: mtd_debug: check amount of data read.

mtd-utils: mtd_debug: check amount of data read.

The kernel refuses to read more data from a MTD device than the device
size. However, mtd_debug does not check the amount of data read as
returned by read(2) and assumes the requested amount is always read when
there is no error. Reading 8M data from a 4M flash chip results in 8M
file containing the flash data at the start.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>