char: ipmi: use DEVICE_ATTR helper macro

Instead of open coding DEVICE_ATTR, use the helper macro
DEVICE_ATTR_RO to replace DEVICE_ATTR with 0444 octal
permissions.

This was detected as a part of checkpatch evaluation
investigating all reports of DEVICE_ATTR_RO warning
type.

Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Message-Id: <20210730062951.84876-1-dwaipayanray1@gmail.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>