firmware: qcom_scm: Check for NULL before dereferencing __scm

The function qcom_scm_get_sec_dump_state dereferences __scm without
checking for NULL and could potentially lead to a NULL pointer exception.
This scenario can happen if the clinet calls qcom_scm_get_sec_dump_state
before qcom_scm driver is even probed. Hence, check for NULL before
dereferencing it.

Change-Id: Ic6f7c2b2d616645ea69a8c594c90dc01d689637f
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>