spi/s3c64xx: Move src_clk to local driver data

The pointer to SPI rate source clock had better be the member of
driver local data structure rather than platform specific.
Also, remove definitions of variable 'sci' that are rendered
useless as a consequence.

Signed-off-by: Jassi Brar <jassi.brar@samsung.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>