spi/s3c24xx: remove unnecessary memset of s3c24xx_spi

Memory for this struct 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>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>