cnss2: Clear dump table only when it is valid

Currently dump table will be clear for SSR case no matter it is valid
or not which may cause double free during SSR. Hence fix it by
clearing dump table only when it is valid.

Change-Id: Ie0945548fa50e0d34192d9b5320b4c365a7c73e1
Signed-off-by: Yue Ma <yuem@codeaurora.org>