Fix warn_unused_result warnings in fsp-leds.c

We now prerror() for cases where we're failing to send responses.
there's likely some better handling of this that can go on,
but at least we'll have a log message now.

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Enable -Werror

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>