i2c: exynos5: don't check for irq 0

platform_get_irq never returns 0. Don't check for this. Make it clear that
the error path always returns a negative error code.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>