The runtime offset has to be added to the memcpy source address and
substracted from the return address. This should have been changed in
a43e2bbc46 which changed from returning the negative runtime offset
into changing the positive runtime offset. Instead a43e2bbc46 only
changed a zero substraction ("subs r4, r0, #0") into a zero addition
("adds r4, r0, #0") which was used as a equal to zero test and changed
nothing. This part is reverted here.

Fixes wrong copy / return locations when setup_c is called with
different runtime and link addresses.

fixes: a43e2bbc46 ("ARM: return positive offset in get_runtime_offset()")

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>