In the scenario where IPA UC ready work is processed after
wlan_ipa_uc_ol_deinit, triggered as part of idle shutdown,
there is a possibility for NULL wbm srng pointer derefernce
when setting the tx doorbell address.
Fix is to avoid scheduling UC op work when deinit is in
progress.
Change-Id: Ifa0908912b77f45b7d93c5f9c2f5aaf9e17685c5
CRs-Fixed: 2807218