spi: spi-s3c64xx: Add missing s3c64xx_ prefix to function names

Add a s3c64xx_ prefix to remaining generic function names so it is clear
the code is part of the driver when grepping or looking at debug logs.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Andi Shyti <andi@etezian.org>
Signed-off-by: Mark Brown <broonie@kernel.org>