firmware: qcom: scm: smc: Handle missing SCM device

In the Linux kernel, the following vulnerability has been resolved: firmware: qcom: scm: smc: Handle missing SCM device Commit ca61d6836e6f ("firmware: qcom: scm: fix a NULL-pointer dereference") makes it explicit that qcom_scm_get_tzmem_pool() can return NULL, therefore its users should handle this.