Fix the result of DFU_OP_WRITE operation in mtd_block_op function
when mtd_lock is not supported (-EOPNOTSUPP) to avoid DFU stack
error on the DFU manifestation of the MTD device, when
dfu_flush_medium_mtd is called.

Without this patch, dfu-util failed on dfuERROR state at the end
of the write operation on the alternate even if MTD write
opeartion is correctly performed.

$> dfu-util -a 3 -D test.bin
....
DFU mode device DFU version 0110
Device returned transfer size 4096
Copying data from PC to DFU device
....
Download	[=========================] 100%       225469 bytes
Download done.
state(10) = dfuERROR, status(14) = Something went wrong,
  but the device does not know what it was Done!

Fixes: 65f3fc18

 ("dfu_mtd: Add provision to unlock mtd device")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Acked-by: Sughosh Ganu <sughosh.ganu@linaro.org>