cmd: fdt: Fix iteration over elements above index 1 in fdt get

Always increment both the iterator and pointer into the string
property value by length of the current element + 1 (to cater
for the string delimiter), otherwise the element extracted from
the string property value would be extracted from an offset that
is multiple of the length of the first element, instead of sum
of element lengths until select index.

This fixes 'fdt get value' operation for index above 1 (counting
from index 0).

Acked-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Fixes: 13982ced ("cmd: fdt: Add support for reading stringlist property values")
Signed-off-by: Marek Vasut <marex@denx.de>