ASoC: wcd_cpe_services: Protect access to the list of services

It is possible that the list of services can be accessed from multiple
contexts/threads and this can result in corruption of the list leading
to crash. Fix this issue by protecting access to the list of services.

CRs-fixed: 851197
Change-Id: Ifec44cc1b0ab95907e9b376e5b217b33e80c5569
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>