i2c: s3c2410: use core helper to mark adapter suspended

Rejecting transfers should be handled by the core. Also, this will
ensure proper locking which was forgotten in this open coded version.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>