libmtd_legacy: don't open device in R/W

On legacy kernels with ROM devices, we can get mtdinfo errors like:

libmtd: error!: cannot open "/dev/mtd4"
        error 13 (Permission denied)
mtdinfo: error!: libmtd failed get MTD device 4 information
         error 13 (Permission denied)

We don't need O_RDRW access for informational ioctls(), so make this
O_RDONLY.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>