sata_mv: silence an uninitialized variable warning

Gcc version 4.6.2-12 complains that if we can't find the "nr-ports"
property in of_property_read_u32_array() then "n_ports" is used
uninitialized.  Let's set it to zero in that case.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>