wlan: Protect DXE register access during SSR

During SSR, DXE registers should not accessed until WCNSS comes out
of reset. Accessing these registers when WCNSS is in shutdown could
cause unclocked register issues.

Change-Id: I4c578e28a64a7d55e0ab853ba2dc8c7ee4cd0bf9
CRs-Fixed: 564064