cmd: fdt: Narrow the check for fdt addr

The current code only checks if the fdt subcommand is fdt addr by checking
whether it starts with 'a'.

Since this is a pretty widely used letter, narrow down that check a bit.

Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>