libata: don't check n_sectors during revalidation if zero

If the initial configuration fails early, n_sectors is left at zero.
Checking against it during revalidation makes retried configuration
fail due to n_sectors mismatch.  Ignore zero n_sectors during
revalidation.

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