mhi: core: Skip RDDM download if execution env is unknown

If mhi_get_exec_env API returns MHI_EE_MAX skip RDDM download.
API returns this value if BHI offset read failed or BHI_EXECENV
register read fails. This indicates the underlying transport is
unresponsive so there is no need to trigger RDDM using SYSERR
or request SOC reset.

Change-Id: I0cc85af84f19b425c94fe8aea987ab03af6de032
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>