remoteproc: Use kstrdup_const() rather than kstrdup()

For cases where @firmware is declared "const char *", use function
kstrdup_const() to avoid needlessly creating another copy on the
heap.

Suggested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20200420231601.16781-2-mathieu.poirier@linaro.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>