soc: qcom: smem: fix qcom_smem_is_available and check if __smem is valid

soc: qcom: smem: fix qcom_smem_is_available and check if __smem is valid

Commit 7a94d5f31b54 ("soc: qcom: smem: better track SMEM uninitialized
state") changed the usage of __smem and init now as an error pointer
instead of NULL.

qcom_smem_is_available() wasn't updated to reflect this change and also
.qcom_smem_remove doesn't reset it on module exit.

Update both entry to reflect new handling of __smem.

Fixes: 7a94d5f31b54 ("soc: qcom: smem: better track SMEM uninitialized state")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/aSAnR3ECa04CoPqp@stanley.mountain/
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20251122185002.26524-1-ansuelsmth@gmail.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>