drivers/pci: Get next capability without passing caps

drivers/pci: Get next capability without passing caps

Modify function pci_find_next_cap_ttl to support returning position
of next capability when size "n" is zero.

That can help caller to get next capability offset if caller just
has a information of current capability offset.

That will be used in a follow-on change.

Signed-off-by: Jiqian Chen <Jiqian.Chen@amd.com>
Acked-by: Jan Beulich <jbeulich@suse.com>