realtek: Fix old compatible for HPE 1920-8G PoE

Supported devices are listed in the metadata as the first part of the
DTS compatible. This normally follows the format "vendor,device".

When updating the device name of the 180W 1920-8G PoE an underscore was
used, instead of a comma, to join the vendor and device name. This will
lead to warnings for users wanting to sysupgrade a device with an older
compatible, as the device's info does not match the one the metadata.

Fixes: 987c96e8 ("realtek: rename hpe,1920-8g-poe to match hardware")
Signed-off-by: Sander Vanheule <sander@svanheule.net>
(cherry picked from commit 6a7fa685)