When sd_init_command() get's a with a unknown req_op() it crashes the
system via BUG().
This makes debugging the actual reason for the broken request
cmd_flags pretty hard as the system is down before it's able to write
out debugging data on the serial console or the trace buffer.
Change the BUG() to a WARN_ON() and return BLKPREP_KILL to fail
gracefully and return an I/O error to the producer of the request.