spi: s3c64xx: simplify if statement in prepare_transfer function

The whole function is inside an 'if' statement
("!is_polling(sdd)").

Check the opposite of that statement at the beginning and exit,
this way we can have one level less of indentation.

Remove the goto paths as they are redundant.

Signed-off-by: Andi Shyti <andi.shyti@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>