libata: fix off-by-one in error categorization

ATA_ECAT_DUBIOUS_BASE was too high by one and thus all DUBIOUS error
categorizations were wrong.  This passed test because only ATA_BUS and
UNK_DEV were used during testing and the ones after them - ATA_BUS and
an overflowed entry - behaved similarly.

This patch fixes the problem by adding DUBIOUS_NONE category and use
it as base.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>