scsi: ufs: update pm_lvl during probe only when current pm_lvl is invalid

As per current code, pm_lvl is read from DT on every probe
during system resume after suspend.

Therefore, rpm_lvl and spm_lvl updated by sysfs will be overwritten
with the pm_lvl read from DT during probe after resume from
system suspend.

To avoid this read pm_lvl from DT only when current pm_lvl
is invalid or during the first ufs probe after boot.

Change-Id: I059d574a81d0e0f7cee8b6492e495819579a2d4c
Signed-off-by: Nitin Rawat <nitirawa@codeaurora.org>