spi/rockchip: remove unnecessary memset of rockchip_spi

Memory for struct rockchip_spi is allocated by spi_alloc_master()
using kzalloc() so it doesn't need to be set to 0 one more time.

Signed-off-by: Alexey Klimov <klimov.linux@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>