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

Fixes: 40289e35 ("firmware: qcom: scm: enable the TZ mem allocator")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20241209-qcom-scm-missing-barriers-and-all-sort-of-srap-v2-4-9061013c8d92@linaro.org


Signed-off-by: Bjorn Andersson <andersson@kernel.org>