i2c: exynos5: fix platform_get_irq error handling

If platform_get_irq returns an error, relay this error to the caller of
the probe function. Don't change all errors to -EINVAL. This breaks the
case where platform_get_irq returns -EPROBE_DEFER.

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