remoteproc: qcom: mdt_loader: Don't overwrite firmware object

The "fw" firmware object is passed from the remoteproc core and should
not be overwritten, as that results in leaked buffers and a double free
of the the last firmware object.

Fixes: 051fb70f ("remoteproc: qcom: Driver for the self-authenticating Hexagon v5")
Cc: stable@vger.kernel.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>