arm: mvebu: Fix return_to_bootrom()

Register r0 should be set to return value 0x0 - NO_ERR.

Set r0 with return value after all registers are restored from the
stack, so that the return value is always correct.

Signed-off-by: Pali Rohár <pali@kernel.org>
Fixes: 944c7a31 ("arm: mvebu: Add option to use UART xmodem protocol via kwboot")
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>