cmd: led: fix led command usage for the LED_SW_BLINK enabled case

CONFIG_LED_BLINK and CONFIG_LED_SW_BLINK can be defined independently.
Led blinking works if any (or both) of them is enabled. Unfortunately
the led command help does not display blinking option if only
CONFIG_LED_SW_BLINK is enabled. This is definitely wrong.

This patch fix an issue.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Alexander Dahl <ada@thorsis.com>