common: cli_hush: avoid dead code

Condition "(value == NULL && ++value == NULL)" actully will
always return false.

Instead, use condition "(value == NULL || *(value + 1) == 0)" to detect
such expression "c=". To "c=", *(value + 1) is 0, so directly return -1,
but not continue.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Rabin Vincent <rabin@rab.in>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>