cmd/fdt: fix uncallable systemsetup command

The function that is processing the 'fdt' parameters is one big
if-else if. In order to be able to type command faster only the first
few letter are checked to know which block of code to execute. For
systemsetup, the block of code that was executed was always the wrong
one and ended up in a failure.

} else if (argv[1][0] == 's') {
    process "fdt set" command
} else if (strncmp(argv[1], "sys", 3) == 0) {
    process "fdt systemsetup" command.
}

When typing "fdt systemsetup", the code that was executed was the code
for "fdt set".

This commit fix this issue by moving the "else if" for systemsetup
before the else if for "fdt set". This allow us to keep compatibility
with any script that make use of "fdt s" to set node values.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Acked-by: Simon Glass <sjg@chromium.org>