drivers: soc: qcom: rimps_memlat: add NULL check for SCMI handle

SCMI handle will remain NULL in case of SCMI failures during
initialization.

Update all store functions to check for NULL before accessing
vendor ops structure.

Change-Id: Ied56f62785579078fb06fb526e29aaef9d45e00d
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>