i2c: exynos5: Fix getting the optional clock

"hsi2c_pclk" clock is optional and may not be present for some SoCs
supported by this driver. Nevertheless, in case the clock is provided
but some error happens during its getting, that error should be handled
properly. Use devm_clk_get_optional() API for that. Also report possible
errors using dev_err_probe() to handle properly -EPROBE_DEFER error (if
clock provider is not ready by the time I2C probe function is executed).

Fixes: 697ad249 ("i2c: exynos5: Add bus clock support")
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Chanho Park <chanho61.park@samsung.com>
[wsa: fixed SHA1 of Fixes tag]
Signed-off-by: Wolfram Sang <wsa@kernel.org>