cmd/led.c: Remove unnecessary check on 'cmd' value

We first check that if argc is less than 2 we return CMD_RET_USAGE.  We
then see if argc is greater than 2 and if so call get_led_cmd() to set
'cmd' and otherwise set it to LEDST_COUNT (which will always be positive
as it's an enum with 0 already assigned).  Therefore the test on if cmd
is less than 0 will always be false and simply be omitted.  Reported by
clang-3.8.

Signed-off-by: Tom Rini <trini@konsulko.com>