libata-scsi: do not respond with "invalid field" for FORMAT UNIT

It does not make sense and is confusing to respond with "Invalid
field in CDB" while we have no support at all implemented for
FORMAT UNIT. It is decent to let it go to the default, which
will respond with "Invalid command operation code" instead.

Signed-off-by: Tom Yan <tom.ty89@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>