phy: qcom-ufs: Correct usage of regulator_get()

When regulator_get() tries to resolve a regulator supply but fail to
find a matching property in DeviceTree it returns a dummy regulator, if
a matching supply is specified but unavailable the regulator core will
return an error.

Based on this we should not ignore errors upon failing to acquire the
optional "vddp-ref-clk" supply.

Reviewed-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Reviewed-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>