dbus-sdr: VR Event Status: Fix mode proprety fetch

The dbus call to fetch the mode proprty was not working due to the
variant return type. Fix it by just targeting string.

Before:
```
ipmitool raw 0x4 0x2b 0xd8
Unable to send RAW command (channel=0x0 netfn=0x4 lun=0x0 cmd=0x2b rsp=0xce): Command response could not be provided
```

Tested:
Working call.
```
ipmitool raw 0x4 0x2b 0xd8
 00 04 00 00 00
```

Change-Id: Ie07deeac676011fa6f0aff190d4ea1d13118c218
Signed-off-by: Willy Tu <wltu@google.com>
1 file changed