remoteproc: return error for bad "recovery" debugfs input

If the value written to the "recovery" debugfs file is not one of
the recognized commands return an error to indicate it's invalid.

Signed-off-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/20200228183359.16229-5-elder@linaro.org


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