ASoC: wcd_cpe_core: Fix handling of CPE SSR event

It is possible that CPE block on the codec might not be enabled and SSR
event is triggerred, in such cases the CPE is not initialized and the
core handle to CPE is expected to be NULL. Fix the CPE SSR handling to
make sure SSR handling does not proceed if the core handle is NULL.

CRs-fixed: 698888
Change-Id: Ie525568765d5fb2a0bc0bf33c6da2a5d1b713d20
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>