firmware: qcom_scm-64: Improve SMC convention detection

Improve the calling convention detection to use qcom_scm_call_atomic
and not blindly assume 32-bit mode if the checks fails.

__qcom_scm_is_call_available() cannot be used directly here as we want
an atomic variant of is_call_available(). This is also a necessary
requirement later when merging in legacy convention.

Change-Id: I66beec550d05afde6c680d256183e8ad207d7268
Signed-off-by: Elliot Berman <eberman@codeaurora.org>