spi: spi-s3c64xx: Use local variable for FIFO length

More references to fifo_len are added in subsequent patch.

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