scsi: ufs-qcom: fix ref-clk pad regulator initialization

UFS device's reference clock is sourced via REF CLK pad on host and this
pad is powered by 1.2v fixed voltage rail. As we don't need to explicitly
set the voltage for this rail, relevant voltage level parameters from
device tree are optional and hence absence of them shouldn't fail the
ref-clk regulator voting from UFS driver.
Fix this by treating the voltage level parameters optional which was
the original intent.

Change-Id: Ic498e5c89a869374bfabfef9ae796a2e78aef834
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>