remoteproc: qcom: hexagon: Clean up mpss validation

As reported by Dan the unsigned "val" can't be negative. But instead
correcting the check for early errors here followed by a wait for the
validation result to show the error or success we can consolidate these
two parts of the validation process into the validation function.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>