spi: spi-s3c64xx: Do not ignore timeout errors in polling I/O mode

Currently timeout errors in polling I/O mode transfer are silently ignored.
Fix it by returning an error when we time out waiting on the RX FIFO level
to reach the transfer length.

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