ata: libahci: Silence compiler warning on 64-bit

Commit 725c7b57 (ata: libahci_platform: move port_map parameters
into the AHCI structure) moves flags into the struct ahci_host_priv's
.flags field, which causes compiler warnings on 64-bit builds when that
value is cast to a void * pointer. Cast to an unsigned long so that the
subsequent cast to a pointer doesn't produce a warning.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>