ARM: at91: Don't invoke spl_boot_device() twice

Since the spl_boot_mode() is now passed the boot device to boot from,
make use of it instead of inquiring for the boot device again. This
allows board_boot_order() to function correctly.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>