drivers: soc: qcom: rimps_memlat: add NULL check for vendor ops

Vendor ops will remain NULL in case of SCMI failures during
initialization.

Update store min and max functions to check for NULL before
accessing vendor ops structure.

Change-Id: I5a93a7161d8d01510f7d580c25e268fdc113df6b
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>