bus: mhi: Fix potential out-of-bound access

In mhi_sat_isvalid_header function if the length is less than
the size of header then there can be out-of-bound access.

So fix the len check in the function.

Change-Id: I80f1556557b1bf2f30c07f6377bd6e3db48712b3
Signed-off-by: Krishna chaitanya chundru <quic_krichai@quicinc.com>