ahci: fix nvec check

commit 17a51f12

 ("ahci: only try to use multi-MSI mode if there is more
than 1 port") lead to a case where nvec isn't initialized before it's
used.  Fix this by moving the check into the n_ports conditional.

Reported-and-reviewed-by Colin Ian King <colin.king@canonical.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tejun Heo <tj@kernel.org>