drivers: sunxi-rsb: fix error output type

"len" is actually a size_t in this function here, so properly annotate
the dev_err printf type to allow compilation for 64-bit architectures.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>