swr-wcd-ctrl: add check for end of soundwire port list

Crash is observed due to out of bound access after
end of list. Add condition to check end of list.

CRs-Fixed: 2046323
Change-Id: Icf01b0265578099813a8572659c985eb4dcd309b
Signed-off-by: Surendar karka <sukark@codeaurora.org>