mtd: mtdconcat: map through panic write handler

Allows a mtdconcat's subdevice->_panic_write to be used for
capturing a mtdoops dump.

Note: The ->_panic_write is mapped through from the first chip
      that is part of the concat virtual device.

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
[miquel.raynal@bootlin.com: return err, not void]
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200602143403.13465-1-matthew.weber@rockwellcollins.com