firmware: qcom: scm: Cleanup global '__scm' on probe failures

In the Linux kernel, the following vulnerability has been resolved:

firmware: qcom: scm: Cleanup global '__scm' on probe failures

If SCM driver fails the probe, it should not leave global '__scm'
variable assigned, because external users of this driver will assume the
probe finished successfully. For example TZMEM parts ('__scm->mempool')
are initialized later in the probe, but users of it (__scm_smc_call())
rely on the '__scm' variable.

This fixes theoretical NULL pointer exception, triggered via introducing
probe deferral in SCM driver with call trace:

qcom_tzmem_alloc+0x70/0x1ac (P)
qcom_tzmem_alloc+0x64/0x1ac (L)
qcom_scm_assign_mem+0x78/0x194
qcom_rmtfs_mem_probe+0x2d4/0x38c
platform_probe+0x68/0xc8