cmd/led: check subcommand "list" instead "l"

current implementation for checking if "led list"
command is called checks only if "l" is passed to the
led command. This prevents switching leds with name
which starts also with a "l". So check for passing
"list".

While at it, also fix a typo in led command usage.

Signed-off-by: Heiko Schocher <hs@denx.de>