ata: sata_mv: check for errors when parsing nr-ports from dt

If the nr-ports property is missing ata_host_alloc_pinfo is called with
n_ports = 0. This results in host->ports[0] = NULL which later makes
mv_init_host() oops when dereferencing this pointer.

Instead be a bit more cooperative and fail the probing with an error
message.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Tejun Heo <tj@kernel.org>