cmd_fdt: save fdtaddr in hex format

Commit 90fbee3e ("cmd_fdt: Actually fix fdt command in sandbox")
changed the format(from hex address to unsigned long) in which "fdtaddr"
is saved . However do_fdt continues reads the "fdtaddr" assuming it to
be in hex format. This may lead to fdt being either loaded or attempted
to load at erroneous address generating fault if the address is out of
memory.

This patch changes back the format to hex while saving the "fdtaddr"
as it was done before.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Hua Yanghao <huayanghao@gmail.com>
Cc: Heiko Schocher <hs@denx.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Simon Glass <sjg@chromium.org>