remoteproc: qcom: mdt_loader: Use signed type for offset

In the transition from using rproc_da_to_va(), the type of the load
offset became unsigned. This causes the subsequent check to let negative
values less than p_memsz + mem_size through and we write outside of the
buffer.

Change the type back to a signed value to catch this.

Fixes: 7f0dd07a ("remoteproc: qcom: mdt_loader: Refactor MDT loader")
Fixes: e7fd2522

 ("remoteproc: qcom: q6v5: Decouple driver from MDT loader")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Acked-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>