bus: mhi: core: Skip RDDM download for unknown execution environment

If MHI is unable to determine the execution environment during
the panic path, host must skip the RDDM download. This can happen
if the BHI offset read or the BHI_EXECENV register read fails
indicating that the underlying transport is unresponsive. Hence,
there is no need to trigger an RDDM using SYSERR or request an
SOC reset.

Suggested-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>