kirkwood: fix device node name of Iomega ix2-200

kirkwood: fix device node name of Iomega ix2-200

The current device node name of ix2-200 is "iom_ix2_200", which results
in a SUPPORTED_DEVICES string "iom,ix2,200" that does not match the
compatible in DTS and the board name used in board.d.

Fix this by replacing the second underscore with a dash, following
vendor_model scheme.

Fixes: 27b2f0fc0fc5 ("kirkwood: add support for Iomega Storcenter ix2-200")
Signed-off-by: Sungbo Eo <mans0n@gorani.run>
[commit title/message rephrase]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>