usb-msctest: Be more conservative setting GETMAXLUN quirk

Only set the GETMAXLUN quirk when it causes an error, but don't set it
when it returns 0. Since we reset the device when we set any quirk, only
set this quirk when it generates an error so umass will avoid it. When
the command works, there's no reason for umass to avoid it at all.

MFC After: 1 week
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