wil6210: prevent unsafe access to stats in wil_sring_reap_rx_edma

In error cases stats can be used before it is initialized.
Initialize it to NULL and check it before usage to guarantee safe
access.

Change-Id: Iedbd10b2dd24d78e901e656bfbdbebce23b51c41
Signed-off-by: Maya Erez <merez@codeaurora.org>