[libata] More robust parsing for IDENTIFY DEVICE multi_count field

Make libata more robust when parsing the multi_count
field from a drive's identify data.  This prevents us from
attempting to use dubious multi_count values ad infinitum.

Reset dev->multi_count to zero and reprobe it each time
through this routine, as it can change on device reset.

Also ensure that the reported "maximum" value is valid
and is a power of two, and that the reported "count" value
is valid and also a power of two.  And that the "count"
value is not greater than the "maximum" value.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>